Projection of metropolitan and eurban Areas

Author: Zhao Bo, author of the projection of metropolitan and eurban areas: October 11, December 26, 2007 • Technology • 9 comments

Http://www.geoinformatics.cn/2007/12/%E5%85%B3%E4%BA%8E%E9%83%BD%E5%B8%82%E5%9C%88%E5%92%8Ce%E9%83%BD%E5%B8%82%E7%9A%84%E6%8A%95%E5%BD%B1%E9%97% AE %E9%A2%98/

Recently, we have made some preliminary research on the 2.5D WebGIS of metropolitan areas and eurban areas. In my opinion, in terms of MAP implementation, this 2.5D map is nothing more than creating a model with 3Dmax or sketchup, giving a perspective, and then rendered to generate a raster image to be displayed on the server. However, such raster images do not have a geographical coordinate system or do not reference map projection. That is to say, if such a map is to be computed geographically, such as the calculation distance and area, it will lead to many problems, or even difficult to do. Therefore, this 2.5D WebGIS still has projection in theory.

So what projection is used for the Metropolitan Area and the emetropolis?

In my first thought, we may use the azimuth projection. We use the azimuth projection at both ends, and some countries with relatively small area also use the azimuth projection. In addition, the research area targeted at 2.5 D WebGIS is also the main city, so it is very likely that it will be azimuth projection. But then I defined Azimuth Projection using ArcGIS and found that the distortion in the edge range is very large, because the point of view of the Azimuth Projection is a point, so her Projection range is a circle, therefore, the distortion in different regions is different.

Later, I thought over it carefully. For example, the projection of a metropolitan area or an ecity should be directed by a parallel light source from infinite distance, and then given an illumination angle, therefore, in theory, we should use orthographic projection ). The experiment is also feasible. Note that you need to set two parameters for Oblique Projection: the longitude center and the latitude center. You need to set them according to the actual situation. At the same time, I have come up with a new idea that we can use different projections for the metropolitan area, and then change on-the-fly, so that we can not only use a single perspective during observation, they have different perspectives. This is a key issue and a convenient conversion process of 2.5-> 3. The projection of my own metropolitan area is shown below:

PROJCS ["orth_dsq", GEOGCS ["GCS_WGS_1984", DATUM ["D_WGS_1984 ",

SPHEROID ["WGS_1984", 6378137.0, 298.257223563],

PRIMEM ["Greenwich", 0.0], UNIT ["Degree", 0.0174532925199433],

PROJECTION ["Orthographic"], PARAMETER ["False_Easting", 0.0], PARAMETER ["False_Northing", 0.0],

PARAMETER ["Longitude_Of_Center", 80.0], PARAMETER ["Latitude_Of_Center",-8.0], UNIT ["Meter", 1.0]

PS: If you want to achieve a large effect, you can consider a perspective projection (perpective projection), which will add a height to the light source.

Related Articles:

• The projection used by google map is EPSG: 900913