**Original address: http://blog.sina.com.cn/s/blog_663d9a1f01017cyz.html**

**1. Dynamic projection (ARCMAP)**

The so-called dynamic projection means that the spatial reference of data in ArcMap or the coordinate system is the coordinate system of the first file loaded into the current workspace, followed by the data, if it is not the same as the current workspace coordinate system, ArcMap will do the projection transformation automatically. After adding the data projection transformation to the current coordinate system display! But the data stored in the file does not change at this time, only the changes in the shape of the display! So it's called dynamic projection! The most obvious example of this is that when you export data, you will be given the choice of whether to press this layer's source data (the data source's coordinate system export) or to export it according to the data (the coordinate system of the current frame).

**2. Coordinate system description (ArcCatalog)**

We all know that in Arccatalog can be a data coordinate system description! That is, the right mouse button on the data->properties->xy coordinate System tab, here can be modify,select, import way to select the coordinate system for the data! But there are a lot of people who think that the data itself has changed after the change. But that's not true! The **information that is abbreviated here corresponds to the. aux file for that data! If you go to delete the file and re-view the file properties, you will still see the unknown! This is just a description** of the data, just like you fill in the basic information registration card, I changed the description but did not change you this person himself! So the coordinate value of the data stored in the data file does not have a real projection transformation to the coordinate system you want to change to! But this description of the data is also very important, if you get a data, from the coordinates shown in ArcMap, like the projection coordinate system under the plane coordinates, but do not know what is based on the projection! So you won't be able to do the data in a non-processing! For example: Projection transformation operation! Because you don't know which projection to start with! So you should correct the understanding of the description of the coordinate system in the data attributes in Arccatalog!

**3. Projection Transformation (ARCTOOLBOX)**

Above said so much, to really change the data how to do, that is, to do the projection transformation! Do it under the Arctoolbox->data Management tools->projections and Transformations! There are a few tools that are most commonly used under this toolset:

**1, Define Projection**

**2, Feature->project**

**3, Raster->project Raster**

**4, Create Custom Geographic transformation**

Displayed as unknown! when the data does not have any spatial reference Use define projection to define a coordinate System for the data, and then use the Feature->project or Raster->project raster tool to transform the data into a projection! Because our country often uses the projection coordinate system to be Beijing 54, Xi ' an 80! When these two coordinate systems are transformed into other coordinate systems, it is usually necessary to provide a geographic transformation, because Datum has changed! Here's what we're talking about. Convert 3 parameters, convert 7 Parameters! And our country's conversion parameters are confidential! So you can calculate it yourself or ask the National Surveying department when you purchase the data! Once you know the conversion parameters, you can use the Create custom Geographic transformation tool to define a geographic transformation method, which can be selected based on the 3 parameter or the 7 parameter according to the Geocentric_translation and Coordinate_ Method! This completes the projection transformation of the data! The coordinates of the data itself have changed! Of course, this projection transformation can also be done in arcmap by changing the coordinate system of data, just after the data can be exported in accordance with the coordinate system!

Method One:

Converting in ArcMap:

1. Load the data to be converted, the lower right corner is the latitude

2. Click View--Data frame properties--coordinate system

3. Import or select the correct coordinate system to determine. The coordinates are also displayed in the lower right corner. But the data didn't change.

4. Right-click Layer--data--Export data

5. Select the second (data frame), output path, OK.

6. This method is similar to a projection transformation.

Method Two:

Converting in Forestar:

1. New layer AA with correct coordinate system and range

2, open the data to be converted, the layer output is the same as the original type, named AA, append.

Method Three:

Converting in Arctoolbox:

1, management tools-projection (project), select the input and output path and the output coordinate system

2, **if the original data must have a projection**

**The difference between geographic coordinate system and projected coordinate system**

1, first understand the geographic coordinate system (geographic coordinate system), geographic coordinate system literal translation is

Geographic coordinate system, which is a storage unit with latitude and longitude as the map. It's obvious that geographic coordinate syst

EM is a spherical coordinate system. We want to store the digital information on the earth on the spherical coordinate system, how to operate

It? The earth is an irregular ellipsoid, how to store data information in a scientific way to the ellipsoid? This inevitably requires

We find such an ellipsoid. Such an ellipsoid has characteristics: it can be quantified and calculated. With a long half shaft, short

Half axis, eccentric heart rate. The following lines are the krasovsky_1940 ellipsoid and its corresponding parameters.

spheroid:krasovsky_1940

Semimajor axis:6378245.000000000000000000

Semiminor axis:6356863.018773047300000000

Inverse flattening (flat rate): 298.300000000000010000

However, with this ellipsoid it is not enough, and a geodetic datum is needed to locate the ellipsoid. In the coordinate system, stroke

In this section, you can see that there is a line:

datum:d_beijing_1954

Indicates that the geodetic plane is d_beijing_1954.

--------------------------------------------------------------------------------

With the spheroid and datum two basic conditions, the geographic coordinate system can be used.

Full parameters:

Alias:

Abbreviation:

Remarks:

Angular Unit:degree (0.017453292519943299)

Prime Meridian (starting longitude): Greenwich (0.000000000000000000)

Datum (earth plane): d_beijing_1954

Spheroid (Reference ellipsoid): krasovsky_1940

Semimajor axis:6378245.000000000000000000

Semiminor axis:6356863.018773047300000000

Inverse flattening:298.300000000000010000

2, Next is projection coordinate system (projection coordinate systems), first look at the projection sitting

Some of the parameters in the standard system.

Projection:gauss_kruger

Parameters:

false_easting:500000.000000

false_northing:0.000000

central_meridian:117.000000

scale_factor:1.000000

latitude_of_origin:0.000000

Linear Unit:meter (1.000000)

Geographic coordinate System:

name:gcs_beijing_1954

Alias:

Abbreviation:

Remarks:

Angular Unit:degree (0.017453292519943299)

Prime Meridian:greenwich (0.000000000000000000)

datum:d_beijing_1954

spheroid:krasovsky_1940

Semimajor axis:6378245.000000000000000000

Semiminor axis:6356863.018773047300000000

Inverse flattening:298.300000000000010000

As can be seen from the parameters, each projected coordinate system is bound to have geographic coordinate systems.

The projection coordinate system is essentially a planar coordinate system whose map units are usually meters. Geographic coordinate system units are degrees

So why are the parameters of the geographic coordinate system in the projection coordinate system?

At this point, it is also important to illustrate the significance of projection: the process of converting spherical coordinates to planar coordinates is called projection.

Well, the projection condition comes out:

A, spherical coordinates

b, conversion process (i.e. algorithm)

In other words, to get the projected coordinates, you have to have a spherical coordinate of the "take" projection, before you can use the algorithm

to the projection!

That is, each projected coordinate system must require a geographic coordinate system parameter.

3, we now see a lot of textbooks on the coordinate system of the address of many, can be attributed to the above two types of investment

Shadow. These include our common "non-earth projection coordinate system". ）：

__________________

Geodetic coordinates (geodetic coordinate): coordinates of the reference ellipsoid in geodetic measurements. The position of the ground point P is indicated by the Earth longitude L, the Earth latitude B and the earth height H. When the point is on the reference ellipsoid, it is represented only by the longitude and latitude of the Earth. The longitude of the earth is through the angle between the meridian plane of the earth and the beginning of the Earth meridian, and the latitude of the earth is the angle between the normal of the point and the equatorial plane, and the earth height is the distance of the ground point along the normal to the reference ellipsoid.

Square Net: is a grid of two sets of parallel lines parallel to the projection axis. Because it is every kilometer to draw the coordinates of the longitudinal line and horizontal lines, so called square in the net, because the square line is also parallel to the Cartesian axis of the coordinate network cable, it is also known as rectangular coordinate network.

On the topographic map of 1:10,000--1:20, the longitude is shown directly in the form of the contour line, and the corresponding degrees are injected at the angle of the figure. In order to encrypt into a net in the diagram, inside and outside the outline is also painted encrypted Graticule encryption sub-line (the scheme is called "the" "sub-band"), if necessary, corresponding to the short-term connection can constitute an encrypted longitude network. 1:2 50,000 topographic map, in addition to the Neatline profile is painted on the latitude and longitude Network encryption division, the figure also has a cross-line encryption.

China's 1:500,000--1:100 topographic map, on the surface directly painted longitude nets, Neatline profile also for the encryption Longitude network encryption sub-line.

The coordinate system of a Cartesian mesh is the x-axis of the line projected by the central Meridian, and the line after the equator is the y-axis, and their intersection point is the origin of the coordinates. In this way, there are four quadrants in the coordinate system. The vertical axis from the equator to the north is positive, negative to the south, the horizontal axis from the central meridian, the east is positive, West is negative.

Although we can assume that the square is rectangular, geodetic coordinates are spherical coordinates. But we often see in a map of the square and latitude and longitude network, we are very accustomed to the longitude and latitude network for the geodetic coordinates, this time the geodetic coordinates are not spherical coordinates, she and the net projection of the square is the same (usually Gaussian), but also the plane coordinates.

**Some understandings about coordinate system (geodetic coordinate, plane coordinate, projection, Beijing 54, Xi ' an 80, WGS84)**

First from the simple point of the assumption that the earth is a circle, the Earth on the surface of a point can be expressed by latitude and longitude, then the latitude and longitude is unique. That is not the only case, that is, the earth is not a round time. The reality is that the earth is not round, but an ellipse. About this ellipse is not unique, such as Krasovsky ellipsoid, 1975 international ellipsoid and so on. The different ellipsoid is mainly represented by two parameters, one is the long half axis, the other is the flattening rate. There will be different ellipsoid, because the earth is too big, the earth is not a normal ellipsoid, an ellipsoid can not meet the accuracy of every corner of the earth requirements, in some marginal area error will be very large, near the equator there is suitable for the equator to use the ellipsoid, near the polar circle has a suitable polar circle ellipsoid, Everything is in order to meet the local precision needs. You can also customize an ellipsoid if you have enough needs. For these reasons, latitude and longitude is not unique, this should be well understood, when you use the Krasovsky ellipsoid is a pair of latitude and longitude, when using another ellipsoid is another pair of latitude and longitude.

**a geographical coordinate system, also known as the geodetic coordinate system, is represented by latitude and longitude** . Sometimes the geographic coordinate system is not convenient, people are more accustomed to using planar coordinate system, planar coordinate system is represented by XY.

It is necessary to turn the coordinates of the sphere surface into planar coordinates, which is called projection. The projection method is also not unique, or for one purpose, to make the local coordinates the most accurate. So at present there are a lot of projection methods, such as Gaussian projection, Mercator projection and so on. Who has the skills and the needs of that area can also create a set of projection methods.

Next is about WGS84 Beijing 54 Xi ' an 80 concept

First there is WGS84 Beijing 54 Xi ' an 80 geodetic coordinate system, is expressed by latitude and longitude, there are WGS84 Beijing 54 Xi ' an 80 plane coordinate system, using XY.

The ellipsoid of the WGS84 uses the international geodetic and Geophysical Union, the 17th session of the General Assembly. Recommended values for measurement constants

Beijing 54 uses the Krasovsky ellipsoid

Xi ' An 80 uses the 1975 international ellipsoid

So the Earth's surface point of these three Earth coordinates is not the same, that is, latitude and longitude is not the same.

At present the more popular is Gauss-gram gauss–krüger projection and Mercator projection, of course also can use other projection, see actually need.

When it comes to different coordinate systems, there is the problem of coordinate turning. **on the coordinate transformation, it is first to understand the problem of the tightness of the conversion, that is, the coordinate transformation in the same ellipsoid is strict, and the conversion between the different ellipsoid is not strict at this** time. For example, the conversion of a Gaussian plane rectangular coordinate with a 1954 Beijing coordinate system to 1954 Beijing coordinates is a coordinate transformation within the same reference ellipsoid, and its conversion process is rigorous. The transformation from the geodetic coordinates of the 1954 Beijing coordinate system to the geodetic coordinates of the WGS-84 is the transformation between the different ellipsoidal bodies.

The common method of using coordinate transformation between different ellipsoid in local area is similar transformation method, that is, the corresponding conversion parameters can be obtained by using the relatively reasonable and high rank common points of partial distribution. In general, the more rigorous is a **seven-parameter** similarity transformation method, namely X-Shift, Y-shift, Z-shift, X-rotation, y-rotation, Z-rotation, scale change K. Seven parameters required in a region need more than 3 known points, if the range is not large, the distance between the farthest point is not greater than 30Km (experience), this can be used three parameters, that is, x translation, y translation, z translation, and x rotation, y rotation, z rotation, scale change K as 0, So the three parameters are just a special case of the seven parameter.

If the influence of elevation is not taken into account, the four-parameter similarity transformation method is used for Gaussian plane Cartesian coordinates under different ellipsoidal bodies, i.e. four parameters (X-shift, Y-shift, scale change m, rotation angle α). If the accuracy of the user requirements is less than 20 meters, within a certain range (2 ' * * *), you can directly use the two-parameter method (ΔB,ΔL) or (δx,δy) correction. In practice, however, this also depends on whether the chosen common point is reasonable and ensures sufficient precision.

**ArcGIS** **the coordinate system definition and projection transformation in**

Coordinate system is the important mathematical foundation of GIS data, it is used to represent the reference system of geographical features, images and observations, and the definition of coordinate system can ensure that geographic data display its position, direction and distance correctly in software, and the lack of GIS data of coordinate system is imperfect. It is therefore important to properly define the coordinate system and perform the projection transformation in the ArcGIS software.

1. Coordinate systems in ArcGIS

Two sets of coordinate systems are predefined in ArcGIS, the geographic coordinate system (geographic coordinate system), and the projected system (Projectedcoordinate system).

1. 1 Geographic coordinate system

Geographic coordinate system (GCS) uses three-dimensional spheres to define the position on the earth. Important parameters in GCs include angular units of measure, Prime Meridian, and Datum plane (based on a spheroid). Latitude and longitude are used in geographic coordinate systems to determine the point on a spherical surface, and latitude is the angle of measurement from the center of the earth to a point on the globe. The horizontal lines in the spherical system are equal latitude lines or parallels, and the vertical line is the longitude line or meridian. These lines envelop the earth and form a grid network called the Graticule.

The longitude and latitude values in GCs are measured in decimal units or in degrees, minutes, and seconds (DMS). Latitude values are measured in relation to the equator, ranging from -90° (Antarctic point) to +90° (North Pole). Longitude values are measured relative to the prime meridian. The range is -180° (when traveling westward) to 180° (when traveling eastward).

In ArcGIS, the coordinate system commonly used in China is gcs_beijing_1954 (krasovsky_1940), gcs_xian_1980 (iag_75), gcs_wgs_1984 (wgs_1984), gcs_cn_2000 (CN _2000).

1.2 Projected coordinate system

The process of converting spherical coordinates to planar coordinates is called projection. The nature of the projected coordinate system is the planar coordinate systems, in which the map units are usually meters. The projected coordinate system is defined in a two-dimensional plane. Unlike geographic coordinate systems, the length, angle, and area of a projected coordinate system are constant within a two-dimensional space. The projected coordinate system is always based on a geographic coordinate system, i.e.:

" **projected coordinate system** **=** **Geographic coordinate system +** **projection algorithm function** ".

Our country's projection coordinate system mainly uses the Gauss-gram gauss–krüger projection, divides into 6 degrees and 3 degrees the projection, 1:25,000 -1:50 The scale topographic map uses the difference 6 degrees the belt, 1:10,000 scales the topographic map uses the difference 3 degrees to divide. The specific zoning method is: 6 degrees from the Prime meridian (Prime Meridian), according to the difference of 6 degrees for a projection belt from the west to the east, a total of 60 projection belts, China across 13-23 bands, 3-degree projection belt from the longitude 1 degrees 30 points longitude (1.5°), According to the deviation of 3 degrees for a projection belt from west to East division, the world is divided into 120 projection belts, China across 25-45 belt.

In the coordinatesystems\projected coordinate Systems\gauss kruger\beijing 1954 directory, we can see four different naming methods:

**Beijing 1954 (Xian 1980) 3 degree GK CM 117E**

Beijing 54 (XI ' an 1980) 3 degrees with no number

**Beijing 1954 (Xian 1980) 3 degree GK Zone 25**

Beijing 54 (XI ' an 1980) 3 degrees with band number

**Beijing 1954 (Xian 1980) GK Zone 13**

Beijing 54 (XI ' an 1980) 6 degrees with band number

**Beijing 1954 GK Zone 13N**

**Xian 1980 GK CM 75E**

Beijing 54 (XI ' an 1980) 6 degrees with no number

Note: GK is a Gaussian gram Gauss–krüger, CM is the Centralmeridian Central Meridian, Zone is a band number, n is a sign that does not show the band number.

2. Defining the coordinate system in ArcGIS

All geographic datasets in ArcGIS require a coordinate system for displaying, measuring, and transforming geographic data, which is used in ArcGIS. If the coordinate system of a dataset is unknown or incorrect, you can use a tool that defines the coordinate systems to specify the correct system coordinates, and you must have learned the correct coordinate system for the dataset before using this tool.

The tool defines a coordinate system for a feature class or DataSet that contains an undefined or unknown coordinate system, located in Arctoolbox-data management tools-projections andtransfomations-define projections

**Input Dataset** **:** to define a dataset or feature class for a projection

**CoordinateSystem** **:** The coordinate system defined for the data set

3. An ArcGIS-based projection transformation

In the operation of data, we often need to transform the data of different coordinate systems into a unified coordinate system, so that the data can be processed and analyzed, the coordinate system transformation in software is commonly used in the following two ways:

3.1 Projection transformation with defined parameters directly

When coordinate transformation parameters are already defined in ArcGIS software, you can directly invoke the coordinate System transformation tool and select the conversion parameters directly. The tool is located in the Arctoolbox-data management tools-projections andtransfomations--feature-project (Raster data projection conversion tool Raster-project Raster), enter the following parameters in the tool interface:

**inputdataset**: A feature class, feature layer, or feature dataset to be projected

**Outputdataset**: A new feature dataset or feature class that has a coordinate system specified in the output coordinate system parameter.

**Out_coor_system**: New coordinate system to which the feature class is known to be converted

**geographictransformation:** in the list is the conversion parameter, with gcs_beijing_1954 to gcs_wgs_1984 as an example, each conversion parameter has the following meanings:

Beijing_1954_to_wgs_1984_1 15918 Ordos Basin

Beijing_1954_to_wgs_1984_2 15919 Yellow Sea sea Area

Beijing_1954_to_wgs_1984_3 15920 South China Sea-Zhujiang Estuary

Beijing_1954_to_wgs_1984_4 15921 Tarim Basin

Beijing_1954_to_wgs_1984_5 15935 Beibu Gulf

Beijing_1954_to_wgs_1984_6 15936 Ordos Basin

3.2 Custom three-parameter or seven-parameter conversion

When the direct conversion between projections is not automatically implemented in ArcGIS software, a custom seven-parameter or three-parameter projection transformation is required, with the seven parameter as an example, the conversion method is as follows:

3.2.1 Custom seven-parameter geo-conversion

In Arctoolbox Select the Create Customgeographic Transformation tool, in the popup window, enter a name for the conversion, such as wgs84tobj54. Under define a geo-conversion method, select the appropriate conversion method, such as Coordinate_frame, in method, and then enter seven parameters, that is, the translation parameter, the rotation angle, and the scale factor:

3.2.2 Projection Transformation

Open the projections and Transformations>feature>projectunder the Toolbox, enter the data you want to convert in the popup window, and output coordinate System, then enter the first step of the custom geographic coordinate system such as wgs84tobj54, start the projection transformation, complete the projection transformation:

Author: Chaolishu e-mail:[email protected]

Projection of the experience 1,

**in different coordinate systems, the latitude and longitude coordinate values of the same point are different**。

2, from 54, 80 to WGS84 conversion parameters are not disclosed, that is, there is no ready-made method for direct conversion. That is, the coordinates of the digital map obtained now are not their self-proclaimed WGS84 coordinates, but the coordinate system of the original map (longitude and latitude values in Beijing 54 or XI ' an 80).

3. The difference between the two maps and the same point is actually the difference between 54 and 80.

** Google maps map projection Full analysis**

Map projections used by web geography such as Google maps and Virtual Earth are often referred to as Web

Mercator or spherical Mercator, the main difference between it and the regular Mercator projection is the Earth

The simulation is a sphere rather than an ellipsoidal body. It is recommended to first make a basic understanding of the map projection knowledge, the map projection

Why. "

**What is the Mercator projection? **

Mercator (Mercator) projection, also known as "conformal positive cylindrical projection", Mercator, Netherlands map historian

(Mercator) was drawn up in 1569, assuming that the earth was enclosed in a hollow cylinder with its equator and circle

And then imagine a lamp in the center of the earth projecting the shape on the sphere onto the cylinder and

The cylinder is unfolded, and this is a "Mercator projection" of the standard parallels for zero (that is, the equator).

Out of the world map. From the ball to the plane, there must be a conversion formula, here is no longer listed.

**Why do Google choose Mercator projection? **

The "conformal" properties of the Mercator projection ensure that the shape of the object is unchanged, and that the square object is projected

Does not turn into a rectangle. The "conformal" also guarantees the correctness of the orientation and the reciprocal position, so that the navigation and

The air is often used, and Google does not make mistakes when calculating the direction of people querying for objects.

The "cylindrical" characteristics of the Mercator projection ensure that the North and south (parallels) and the East and West (meridians) are parallel lines,

and perpendicular to each other. And the warp interval is the same, with the parallels spaced from the standard parallels (here the equator, also

May be other parallels) gradually increasing to level two.

However, the "conformal" inevitably brings large deformations of the area, especially in the polar regions, as evident as

The Greenland is enlarged N times than the actual area. But if you go to the polar regions for exploration or reliable comrades, a

Like there is more detailed information, not to view the network map, this does not matter.

**why a circular sphere, not an ellipsoid?**

This is simple, simply because of the convenience of the implementation, and the calculation of the simple, the accuracy of theoretical differences 0.33%

, especially the larger the scale, the more detailed the figures, the difference can be ignored.

Web Mercator projected coordinate system:

With the entire world, the equator as the standard parallels, the Prime Meridian as the Central Meridian, the intersection of the two are seated

The origin point, east to north is positive, west to south is negative.

X-axis: Because the equatorial radius is 6378137 meters, the equatorial perimeter is 2*pi*r = 20037508.3427892,

So the value range of the X-axis is: [ -20037508.3427892,20037508.3427892].

Y-axis: The formula for projection by Mercator is known, and there is also a schematic, when latitude φ approaches the poles, i.e.

At 90°, the Y value tends to infinity. These are the "lazy engineers" that limit the range of the Y-axis.

Set in between [ -20037508.3427892,20037508.3427892], make a square.

The benefits of lazy people, well-known, pre-cut the static picture, improve access efficiency and so on. I'm just telling you.

Why this is the way it is. So the range under the projected coordinate system (m) is: Minimum

(-20037508.3427892,-20037508.3427892) to Maximum (20037508.3427892,

20037508.3427892).

The corresponding geographic coordinate system:

According to the truth, the geographical coordinate system is the first, such as the sphere or ellipsoid is a geographical coordinate system, and the ink

The Cato projection is not very relevant. In simple terms, the projected coordinate system (PROJCS) is a planar coordinate system to Miwei

The geographic coordinate system (GEOGCS) is the ellipsoid coordinate system, in the latitude and longitude of the unit. For details, refer to the sitting

Standard, coordinate reference system, coordinate transformation, projection transformation.

Longitude: There's no problem here. Global reach: [-180,180].

Latitude: It is known that latitude cannot reach 90°, and lazy people take it for a square.

-20037508.3427892, after the inverse calculation, you can get latitude 85.05112877980659. So latitude

The value range is [ -85.05112877980659,85.05112877980659]. What about the rest of the area?

Yes, the Penguins don't care.

Therefore, the geographic coordinate system (latitude and longitude) corresponds to a range of: min ( -180,-85.05112877980659),

Maximum (180, 85.05112877980659). As for the Datum, coordinate conversion and so on will no longer speak.

Related coordinate calculation:

About the organization of Google maps--Map tile pyramid, I guess I'll repeat it here.

The thing, afraid is also no one to see. Although the principle is the same, but specific to write different manufacturers of different data sources of generation

Code, you will find that the number of zoom levels is different, the minimum level is different, the encoding method is different,

such as Google's Qrst, Microsoft's four fork tree, OSGeo's TMS and so on.

However, you may not have to be so troublesome, because these algorithms are already spread across the web, and you can do it from

Get it from someone else's blog, or learn from open source software. This is not a secret in itself, and Microsoft itself is a public

The cloth.

"Tilesàla Google Maps" is an interactive way to get the boundary range of any Tile,

Popular coding methods. The link to this page is very valuable, and part of this article is an important reference for writing.

The author uses Python to complete the conversion algorithm between the following coordinates: latitude (the coordinates that appear in the KML,

WMS BBOX parameters, etc.), planar coordinates XY (m, Web Mercator projected coordinate system), pyramid

XYZ (i.e. position of X axis, position of Y axis, and zoom level zoomlevel), each Tile

Code key value (Qrst or 0123, etc.). There are two concepts to be aware of when converting, Ground Resolution

and Map scale.

Ground Resolution, ground resolution, like spatial Resolution (spatial resolution),

Here we focus primarily on the form represented by the pixel size: a pixel (pixel) represents the ground

Face Size (m). Take Virtual Earth For example, level is 1 o'clock and the image size is 512*512 (4

Tile), then the equatorial space resolution is: equatorial circumference/512. The spatial resolution of other latitudes is

For the latitude ring length/512, the extreme Arctic is 0. At Level 2 o'clock, the spatial resolution of the equator is red

Road circumference/1024, other latitude is latitude circle length 1024. Obviously, Ground Resolution depends on

At two parameters, the zoom level levels and latitude latitude, which determines the number of pixels, latitude

Determine the length of the ground distance. The formula for the ground resolution is: M/pixel:

Ground Resolution = (cos (latitude * pi/180) * 2 * pi * 6378137 meters)

/(* 2levelpixels)

Map scale, that is, maps scales, elementary school knowledge, on the chart distance than the field distance, both units are generally

It's rice. In the calculation of Ground Resolution, the pixel size of a picture is obtained by level, then

You need to convert it to a distance in meters, which involves a DPI (dot per inch) that can be understood temporarily

For a similar PPI (Pixelper inch), which is the number of pixels per inch. 2LEVEL/DPI *

You get the inches inch, and then you divide the inches inch by 0.0254 into meters. Field distance still

Yes: cos (latitude * pi/180) * 2 * pi * 6378137 meters; So the formula for the scale

For, is generally converted into 1:xxx, no unit:

Map scale = 2level/screen dpi/0.0254/(cos (latitude * pi/180)

* 2 * pi * 6378137)

= 1: (cos (latitude * pi/180) * 2 * pi * 6378137 * screen dpi)/(256 *

2level * 0.0254)

In fact, map scale and Ground Resolution there is a correspondence, after all, and the field distance phase

Association, Relationship: Map scale = 1:ground Resolution * Screen dpi/0.0254

Meters/inch

Virtual Earth Tile System lists the Virtual Earth at the equator, level, pixel

The corresponding relationship between the number, the ground resolution and the map scale, and the paper briefly introduces the Mercator investment

And the above two concepts, recommended.

In addition, "addressing Google Maps image Tiles" application, input latitude and longitude and zoom

level, you can zoom to the appropriate Google Maps location and display the Qrst of the lookup process.

JavaScript implementation of the algorithm can also be captured and "Tilesàla Google Maps" In contrast,

Conversion from latitude to longitude to Tile encoding.

WKT form Representation

Google Maps and Virtual Earth are less popular, but the Web they use

Mercator or spherical Mercator has not been EPSG projection data for a long time

Accepted by the library. EPSG that it does not count as a scientific projection, so it just gives a EPSG:

The marking of 900913 (SRID), this marking is free of EPSG the conventional marking range. (EPSG, SRID

What is it? See also "EPSG, SRID". ）

By the May 2008 (according to Sharpgis classmate), EPSG suddenly understood, whether the spheroid or the sphere,

In fact, the simulation of the Earth, but the exact degree of difference, there is no difference in nature. Or is not

To accept this projection, to define the name of the projected coordinate system PROJCS, without accepting a wide range of factual criteria

For "Popular visualisation Crs/mercator", SRID is epsg:3785; geographical coordinates

The Geogcs is named "Popular visualisation CRS" and the SRID is "Epsg:4055″."

These markings have entered the "normal range". (PS: Is this visualisation English style?) ）

PROJCS WKT "Well known Text" is written as follows, Geogcs, Datum and other WKT

See "Spherical/web MERCATOR:EPSG code 3785". Incidentally, Web Mercator.

The ESRI company number (ESRI calls it well known ID?). ) is 102113 temporarily, perhaps occasionally with

Get.

projcs["Popular visualisation Crs/mercator",

geogcs["Popular visualisation CRS",

datum["Popular_visualisation_datum",

spheroid["Popular visualisation Sphere", 6378137, 0,

authority["EPSG", "7059"]],

towgs84[0,0,0,0,0,0,0],

authority["EPSG", "6055"]],

primem["Greenwich", 0,

authority["EPSG", "8901"]],

unit["Degree", 0.01745329251994328,

authority["EPSG", "9122"]],

authority["EPSG", "4055"]],

unit["metre", 1,

authority["EPSG", "9001"]],

projection["MERCATOR_1SP"],

parameter["Central_meridian", 0],

parameter["Scale_factor", 1],

parameter["False_easting", 0],

parameter["False_northing", 0],

authority["EPSG", "3785"],

axis["X", EAST],

axis["Y", North]]

PostScript: This problem is an old problem, the cost of so much time, the main is to share, after all, they are still a phase

When understood. Also see people do not understand nonsense, write an article corrected. Of course, everyone makes mistakes, including me.

If this article is 100% correct, you can also question it. This topic is not really intended, because it is not scientific,

Even EPSG's Information_source field is written by Microsoft, but the domestic Google

More fire, SEO a bit.

In addition to the links listed in this article, Microsoft, Google, EPSG, OGC, and other organizations

Charlie Savage, Sharpgis, Nelson John and other blogs are also very important.

SOURCE, thank you for this.

(ext.) The definition of coordinate transformations and geographic coordinates and projected coordinates in ArcGIS