Can't Accurately Calculate Area of Polygons I Draw?

[kriselise322]

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?

[DanPatterson]

If your coordinate system is web mercator the areas will be wrong.  I think google maps uses web mercator, so make sure your map isn't either.

Set it to a planar projected coordinate system, state plane or UTM and make sure the units are known (meters or feet) when you are calculating the area.

Also, what do you think the area should be and what is being reported to rule out a feet vs meter issue

[kriselise322]

Hi, thank you so much for your response! Does that mean google maps area calculations would be wrong too? I wonder if the people I'm getting the square footage from online used google maps as their source? I've gone to map -> properties -> coordinate systems -> and tried both a state plane and UTM coordinate system. After doing this and re-running the calculate geometry, using the coordinate system of the map, the calculated area remains unchanged. So regardless of the projection of the map, the area is being calculated the same. The area is calculated as 82,600 square feet, and the area listed online is 84,000 square feet. Is it possible this is a small enough area that there wouldn't be enough distortion to affect the calculation, and the listed area online is just incorrect? But if that's the case, why is the area calculated with google maps so wrong? I can't wrap my head around this. Thanks again for your help.

[DanPatterson]

for best results use an equal area or conformal projection (eg utm, state plane... never web mercator)

use geodesic area !!

Calculate Geometry Attributes (Data Management)—ArcGIS Pro | Documentation

don't get caught up in the US vs international foot thing as well  a small but important issue which could account for differences in area  (best to compare to metric calculations)

In order to wrap your head around all this, just remember that Earth isn't flat and it isn't a perfect sphere and map projections and coordinate systems are an attempt to deal with this. your head will hurt less

metric

[kriselise322]

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

[David_Brooks]

@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

[kriselise322]

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?