After my study on map projection I realized that it is very difficult to choose a projection that allows to compute simultaneously the distances between a great deal of points if these points are located all over the world. Equidistant projections allows only to compute distance between couple of points. Then I decided to use the following solution. My data set can be split in smaller datasets:
- United States territory;
- the whole EURASIA;
- the territory encompassing the African continent and the Europe.
In ArcGIS 9.3 (ArcInfo license) I projected each dataset with the Lambert Azimuthal Equal Area projection properly “centered” for each case. For each dataset I loaded a new layer with the Tissot's Indicatrix shapefile and computed the areas of each ellipse. I considered as "not distorted" (as regards the area, and the distances) all that territories over which the ellipses’ areas do not differ more than the 5% from the ellipse nearest to the center of the projection and when the ellipses' shape do not reasonably differ from a circular one. After having seen that my points were enclosed in this “not distorted” territory, I computed the point distance. Do you think my procedure was reasonably correct or have some advice about a better map projection for my purpose?
Answer
If you're only concerned with calculating distances between two points on the Earth's surface, you don't really need to work this out through maps and map projections.
What you need is a formula for calculating geographical distances: http://en.wikipedia.org/wiki/Geographical_distance
If you need high accuracy, look for http://en.wikipedia.org/wiki/Geographical_distance#Ellipsoidal-surface_formulae
No comments:
Post a Comment