topic Calculating Area in Map Projections Questions

Calculating Area

MatthewWilley — Mon, 12 Jan 2015 13:44:19 GMT

I am working with some worldwide data sets and having issues calculating potential area collection. I am trying to calculate daily potential collection access. I am starting with a world country shape, clipping with daily access and then dissolving to a single feature within the feature class. All have been projected to a meter linear unit projection but I am receiving an answer of over 380 million SqKm. Some of the shapes seem to have the right area but others are several times larger then they should be.

Any advise?

Re: Calculating Area

MelitaKennedy — Mon, 12 Jan 2015 18:36:16 GMT

Which projected coordinate reference system? If it's not equal area, then you may be running in to area distortion that's due to the map projection. It's not Web Mercator, is it?

Melita

Re: Calculating Area

MatthewWilley — Mon, 12 Jan 2015 18:41:19 GMT

I have projected both the input feature and the clipping feature to WGS 84 Mercator for the meter linear unit because the requested output is for SqKm. The weird thing is some of the features are coming out as expected but others are as much 3 or 4 times the total SqKm of the entire shape even though the clip can be less then 1% of the total possible area.

Re: Calculating Area

MelitaKennedy — Mon, 12 Jan 2015 18:50:23 GMT

Mercator is a conformal project so it maintains shapes, not areas. Do you see better area values closer to the equator?

Try using Hammer-Aitoff or Sinusoidal. Most of the equal-area map projections that show the entire world only support spheres for the Earth model. These two do support ellipsoids (spheroids).

Melita

Re: Calculating Area

MatthewWilley — Mon, 12 Jan 2015 18:59:34 GMT

I am actually seeing the most issues in small countries near the equator, For example I have access to about 10% of Guatamala for a specific day but the calculated access is over 280,00SqKm which is 2x the entire country. But my estimated access for Russia is about what I was expecting.

And thank you for pointing out the conformal projection. Not sure why that didnt occur to me earlier.