WGS 1984 web-Mercator for online map service

WGS 1984 is one of the commonly used coordinate systems for 3 S applications and is closely related to our daily lives. The most typical application is the GPS module of handheld devices, the GPS Global Satellite Positioning System of the US, GLONASS of Russia, Galileo of the European Union, and Beidou of China all provide Positioning Based on the WGS 1984 coordinate system, in the unit of minute-second, therefore, many spatial data are based on the coordinate system, which allows you to conveniently overlay and analyze GPS data.

In WebGIS applications, the "GPS + basic map service" application mode is most convenient to use the WGS 1984 coordinate system, but more often, especially when a map contains a small scale, we need to projection the map to a plane. Otherwise, we can only display the world map with a sphere like Google Earth. A conventional projection method is the projection of a positive axis and an equidistance column. The definition on Baidu encyclopedia is as follows: The axial equi-distance cylindrical projection is also called "square projection" and "square projection ". A type of cylindrical projection. The cylindrical projection plane is tangent to the equator. the latitude and longitude network is projected to the cylindrical surface based on the constant length of the Meridian Line, and flattened along a bus. In such a projection chart, the weft is a set of parallel lines with an equal distance between the weft and the field. The warp is a parallel line perpendicular to the weft, And the meridian is equal to the field on the equator, the longer the distance from the equator, the longer the longitude line and the weft line form a grid with a square shape (rectangle. There is no length deformation along the meridian line. The deformation line of the area and angle is parallel to the weft line, and the deformation value increases from the equator to the high latitude. Maps that extend along the equator or low latitude. If you don't understand it, you can just take an experiment with oranges. A typical example is the online map service on ArcGIS online in the early stage: Http://server.arcgisonline.com/ArcGIS/rest/services/ESRI_StreetMap_World_2D/MapServer? F = jsapi The teacher in cartography said that any projection method has advantages and disadvantages. It is applied in the global (national or provincial) basic map service or public basic map service, we will find that there are two hidden problems in it, so that the positive axis is equal to the cylindrical projection advantages are not painful, or even become a disadvantage: 1. Use its advantages to select regions around the equator in the study area. For the Global Map Service, the Equator region is only a small part of it. For the public basic map service, the application of the public basic map service only needs low-latitude areas and is very limited. The angle of the map is too big to be correctly determined between the features. 2. the warp is gradually deformed, And the weft distance and field length are equal. When the full screen is displayed, the map can occupy the screen space to the maximum extent (the aspect ratio of the map is), resulting in more data and hard disk storage space. We like to watch American movies and high-definition movies with wide screens. The length of the player is full of the entire display and the visual effect is comfortable. However, for the map service, it means a large amount of map data. Taking the world map service as an example, we can imagine that at the same scale, if the map is projected on a square (the map aspect ratio is ), the map service can also display the world. If the upper and lower width of a map is full, a blank area will be left and right of the map, saving half of the space compared with the former. Some people may wonder if the hard disk is getting cheaper and the storage volume is getting bigger and bigger. Is there a storage problem? For general applications, you do not have to worry too much, but for the public basic map service, the aspect ratio changes from to, which is like a butterfly effect, which will bring a huge amount of data at a large scale. For example, a WGS 1984 coordinate system and a 512 × 512 cut graph are usually used to generate 14th images in l13 (Level 1,237,819, in the WGS 1984 web Mercator projection coordinate system with the same image size, the number of l13 images will be reduced by half. It is tempting to cut the number of small images by half, other methods can be used for further optimization. This article is only introduced here after we have said so much.ArticleTheme-web Mercator projection, which is currently used by mainstream online maps, such as Google Map, Bing map, and mapabc. Why is this projection method used? 1. The scope of the Web Mercator projection map is half the size, saving 50% space. 2. web-based applications need to focus on the user experience of the client. Large image slices may cause slow transmission, and images in large areas cannot respond in a timely manner. Therefore, you can improve the user experience by reducing the size of image slices. When a 256 × 256 cut graph is used, the number of images will increase by 4 times. When the first point is integrated, the overall number of images will increase by 2 times. When the positive axis is projected by an equi-distance cylindrical, slice with a size of 256x256 will increase the number of images by eight times compared to Web Mercator! 3. The web Mercator projection map is greatly deformed at the two poles and cannot display map information at high latitudes. This is negligible in the application of the public map service, People in the Antarctic and Arctic do not care about your forgetting about them. However, these defects ensure the correctness of the map direction and locations. They are often used in navigation and air, and users will not make mistakes when querying the direction of the map. ESRI added a series of web Mercator projection map services on ArcGIS online. There is another reason: 4. mashup with mainstream map services So with the current global online map service: Http://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer? F = jsapi More map services: Http://blogs.esri.com/Support/blogs/arcgisonline/archive/2009/12/22/arcgis-online-maps-updated-and-migrated-to-google-maps-bing-maps-tiling-scheme.aspx WGS 1984 web Mercator has the following meanings: In the same case, the map has a smaller range, reducing the number of images ==> 256 × 256 slice size can be used to increase the number of images, but the network user experience is improved; Ensure the correctness of the map direction and locations; Mainstream map services mutual mashup