Can't Accurately Calculate Area of Polygons I Draw?

Question

Hi everyone! I'm a relatively new user and I'm having some trouble. I think it's an issue with the projection or something but I just don't know what to do. I have created a new layer, am adding polygons to it that I draw myself using the Imagery basemap, and trying to use calculate geometry to find the area in feet. Some of the polygons have the area listed online so I can reference what they should be calculated as, and they are incorrect, and my calculations are turning up something different from using the calculate area tool on google maps. I don't know what to do. I've tried changing the projection by clicking map -> properties -> [state plane projection for the area], and then re-running the calculation, and it doesn't change at all. What am I missing? Anyone know?

kriselise322 · Answer

Hi David, thanks for your response!

My map is a bit closer to the equator than the UK, but still a ways out-- I'm looking at California's Bay Area (between ~36-38 degrees latitude), so there should still be some distortion.

I've read online that Google uses web mercator and so might have some issues with calculating area; does the geodesic area calculation account for that? Do you think if I draw polygons myself and add them to a layer, project that layer using an equal area/conformal projection, and run the calculate geometry tool using geodesic area, the area will be more or less correct?

kriselise322 · Answer

Thank you for your help! It's very much appreciated!

David_Brooks · Answer

@kriselise322 the issue Dan mentions is due to that fact that in Pro, geometry calculations default to planar mode rather than geodesic. So errors in length and area will increase as a function of your latitude and distance from the equator. Not sure where you are, but in UK latitudes we expect web mercator calcs to be ~1.6x out.

This isn't applicable in google maps because the measurement mode is geodesic I believe, so the areas don't distort depending on latitude.

If your error is only 1-2% out, then I would hazard a guess that you're either very close to the equator, or you are using the correct measurements of area and this is in fact rounding error in the online listing and you've nothing to worry about.

But as Dan says, best to avoid WMAS like the plague