No need for projections?

Question

Simple question: If ArcPro can finally calculate area and distance geodesically, why do we need projections? Why not do all analyses in geographic coordinates and remove the distortion from flattening the curve??

When are projected measurements ever going to be more accurate than geodesically derived areas and distances?

Bojan≈†avriƒç · Answer

I would only add one more detail to Margaret Maher's response. When you look data in geographic coordinate system, it is displayed in pseudo-Plate Carrée projection. You still see distortion of the areas, especially at higher latitudes. World is rounded, but our screens are still flat and displaying data on the screen is the same issue as projecting data on the flat sheet of paper. I hope this makes sense.

MargaretMaher · Answer

Shannon, as Adrian said this is an excellent question, but we have to think about the geometry of the earth.  Standing on what appears to be a flat plane, the horizon is about 2 1/2 miles away in all directions.  Since within that "buffer" distance the curvature of the earth is not relevant to distance or area calculations, a geodesic distance or area would not be appreciably different than a planar distance or area,  depending on the projected coordinate system applied.

In addition, when a surveyor is working on a project, collecting measurements for lots in a new subdivision, or an engineering firm is working on a road project, collecting distances in degrees in not really practical.

The other issue is that a degree on the ground at the equator or north-south, is approximately 69.2 miles or 111 kilometers.  A degree of Longitude on the other hand is variable depending on the distance from the Equator. Using the calculator at the link below, you can check the length of a degree of Latitude in comparison with the length of a degree of Longitude at a specific north or south Latitude location.

http://www.csgnetwork.com/degreelenllavcalc.html

In order to maintain maximum accuracy in coordinates in decimal degrees, you would have to capture the coordinates out to more than 11 places to the right of the decimal.  With a projected coordinate system you can obtain distances and areas to centimeter or inch level.

You might want to check out Esri Knowledge article 000006113 at the link below.  The article includes information about the properties of projections, and includes a link to a list of supported map projections and their properties in Esri software.

https://support.esri.com/en/technical-article/000006113

AdrianWelsh · Answer

Shannon, this is a great question! I think that by adding this to the Coordinate Reference Systems‌ and map projections‌ groups, it will get more visibility. Maybe even tagging Margaret Maher‌ and Melita Kennedy‌ will help too!