Feeling of GIS map projection

From: http://blog.csdn.net/fenghuayoushi/article/details/6095043

Generally, the GIS (Geographic Information System, referred to as "geographic information") and surveying and mapping majors in colleges and universities will learn a specialized course-map projection.

For trusted students, if they do not have a clear concept of map projection, it is not a rigorous academic attitude to learn other aspects, such as interpretation, spatial analysis, and professional software, or an irresponsible attitude. Because map projection is the foundation of GIS, incorrect projection will lead to deviations in almost all subsequent results. The simplest is the area and length statistics. incorrect projection settings will lead to large errors for these basic elements, the analysis is then passed to the subsequent analysis based on these basic elements (such as landscape analysis landscapeanalysis and image classification ). In addition, due to the lack of practical application tests and the inability to detect the problems caused by projection, many students neglect map projection.

If the students of telecommunications and surveying and mapping, especially those of geographic information, can correctly select, set, and convert projections when performing professional operations and analysis, it will be of great help to develop necessary rigorous study habits in the future and lay the most practical foundation for future work and research.

Let's take a look at some of the accumulation points below, which will be constantly modified. There will also be a unified idea in the future:

Hope you can correct me more!

 

1. The geographical coordinate system, the projection coordinate system, and the spatial coordinate system are confusing concepts (in fact, they are easy to understand)

# Spatial coordinate system (3-dimensional coordinate system, equivalent to mathematics), generally refers to a pure three-dimensional coordinate system. The representation methods include X, Y, zcoordinate, and a, B, and r polar coordinates, is the concept in mathematics;

# Geographic coordinate system (three-dimensional, which is suitable for the earth's surface). Since there are two geographical words, it must be related to the geographical things, first, we need to know the purpose of the geographic coordinate system (to locate the location of geographical elements such as cities on the earth's surface). Its form is the same as that of space ,, however, because the earth's surface height is different, the gravity field is different, and the ocean and land are different in nature, the location in the earth is just to deal with the ball center, origin, and radius. It is not difficult to locate a location, but it is difficult to locate a region (because it will produce different degrees of error when it is projected onto a two-dimensional plane, we need to minimize this error ). Generally, the following elements are required for a geographic coordinate system: the replacement of the Earth that simulates the earth surface or the land surface of some countries in a certain area-the elliptical body, must have a unified benchmark as the benchmark for the height start point, it is also because of different types of elliptical bodies (the long axis, short axis, and flat rate vary) the surface fitting effects for each country or region are different (which leads to the difference between the simulation effect and the error when the projection is plane and the actual factors such as the actual gravity magnetic field ), it is possible that this elliptical body is very suitable for the simulation of the actual situation in this country, but there will be a great error in other countries.

# Projection coordinate system (a two-dimensional coordinate system after the three-dimensional coordinate system is projected according to a specific method). Similarly, after the elliptical body is determined, it must be projected to a two-dimensional plane to facilitate our daily viewing and analysis, in this case, what projection method can better reflect the actual ground conditions on the plane? The same as determining the elliptical body-different projection methods have different effects on different regions, so which one is the best? If you're not sure, There will be so many projections.

 

2. The conversion between the WGS84 geographic coordinate system and Beijing 1954 and Xi'an 1980 (this involves the issue of GPS Positioning and projection based on the actual situation in China)

# WGS84 (World Geodetic System 1984) is a geographic coordinate system established for the use of GPS. For details, see the WGS84 entries in different encyclopedias), Beijing 1954, and Xi'an 1980. However, you must note that: ---- (excerpt, we often hope to convert the geographic coordinate system in beijing 54 or Xi'an 80 to WGS84 Through coordinate transformation. In fact, this is not accurate, the projection coordinates of beijing 54 or Xi'an 80 can be computed and converted to the corresponding geographic coordinate system. However, since the conversion parameters from the geographic coordinate system of beijing 54 and Xi'an 80 to WGS84 are not disclosed, therefore, it is impossible to accurately calculate the coordinates from WGS84. For this reason, when ArcGIS is switched from WGS84 to the beijing54 geographic coordinate system, six conversion options are displayed:

Only some empirical data can be used to simulate the conversion. This is the troublesome result caused by no public conversion parameters.

 

3. Pay attention to the split band of projection (mainly horizontal axis cylindrical projection will have a split band) (the starting position of the split band, the degree of the split band is 3 ° or 6 °, in the south or northern hemisphere, and whether there is a belt number at the beginning) and the default offset (China will usually offset 500 km). Of course, we also need to identify the basic concepts of central longitude, standard weft, cut cylindrical or cut cylindrical.

 

4. Problem: ArcGIS image reference system and projection display

Wgs_1984_utm_zone_49n: and

Wgs_1984:

Why is the geographic coordinate system only available in wgs1984? Shouldn't there be a projection? Or is ArcGIS displayed in some projection mode by default?

---------- For this question, I finally know why. By default, ArcGIS projects a common linear projection to these geographic coordinate systems. ArcGIS officially named this normal projection platecarree. The projection calculation is: the longitude and latitude multiplied by the same coefficient is the coordinate value of the plane. It is very simple and the easiest. If it is a world-wide network, the projection will look like a square grid with the same latitude and longitude distance, the most intuitive. ArcGIS uses this so-called projection as the projection of geographical coordinates without projection by default, because such calculation is the most convenient and the intuition is not too bad, I personally think.

 

Refer:

1. Http://bbs.godeyes.cn/showtopic.aspx? Forumid = 111 & topicid = 369678 & forumpage = 1 & go = Prev
2. Http://topic.csdn.net/u/20091202/20/b1335b59-5644-4d87-aeec-3515385cb4da.html