topic Re: determine area of polygon in "EPSG:3857" in ArcGIS API for Python Questions

determine area of polygon in "EPSG:3857"

artzaifman — Sun, 12 Dec 2021 09:28:40 GMT

I downloaded the US states shapefile from census.gov and imported "Texas" as follows:

geom = geometry.Geometry.from_shapely( us_states_.geometry[11], spatial_reference={"wkid": 3857} ) geom.area

956858520477.5554

geom = geomeUsing arcpy method "get_area" gave different answer:

geom.get_area("GEODESIC", units="SQUAREMETERS")

692662873864.4374

Main question is following: is it possible to have "geom.area" return value = "geom.get_area" return value?

Thank you!

Re: determine area of polygon in "EPSG:3857"

artzaifman — Sun, 12 Dec 2021 09:28:43 GMT

Reformatted errors in previous post:

I downloaded the US states shapefile from census.gov and imported "Texas" as follows:

geom = geometry.Geometry.from_shapely( us_states_.geometry[11], spatial_reference={"wkid": 3857} ) geom.area

956858520477.5554

Using arcpy method "get_area" gave different answer:

geom.get_area("GEODESIC", units="SQUAREMETERS")

692662873864.4374  # This is correct!

Main question is following: is it possible to have "geom.area" return value = "geom.get_area" return value?

Thank you!

Re: determine area of polygon in "EPSG:3857"

JoshuaBixby — Mon, 11 May 2020 16:27:42 GMT

Short answer, no. When looking at the arcgis.geometry.Geometry — arcgis 1.8.0 documentation:

propertyarea
The area of a polygon feature. None for all other feature types. The area is in the units of the spatial reference.

get_area(method, units=None)
Returns the area of the feature using a measurement type.

You are downloading the geometry data into WKID 3857, so that is the spatial reference that Geometry.area will report its value in.

Re: determine area of polygon in "EPSG:3857"

artzaifman — Mon, 11 May 2020 16:44:12 GMT

Thanks Joshua. Is there additional information available that would explain how to convert from spatial reference unit area to measurement type?

Re: determine area of polygon in "EPSG:3857"

JoshuaBixby — Mon, 11 May 2020 17:27:48 GMT

You seem to be getting the answer you are interested in using Geometry.get_area, so why not just use that?

Re: determine area of polygon in "EPSG:3857"

artzaifman — Mon, 11 May 2020 17:47:53 GMT

Only reason is "arcpy" is Windows and my development environment is Mac. I'm running "parallels" so that I can access both simultaneously.

Re: determine area of polygon in "EPSG:3857"

artzaifman — Mon, 11 May 2020 18:25:42 GMT

Does it boil down to finding the best projection (with least distortion) when using "area" property as opposed to "get_area()" method in arcpy?

Re: determine area of polygon in "EPSG:3857"

JoshuaBixby — Mon, 11 May 2020 20:04:23 GMT

Absolutely, that is if you want to rely on using Geometry.area . WKID 3857, like all conformal projections, is focused on maintaining direction and shape. Area and distance measurements can be very off, as you are seeing with TX. I think an old standby would be Albers Equal Area Conic USA (102003), but some of the new whole-earth projections from Esri work well like Equal Earth Americas (8858).

Re: determine area of polygon in "EPSG:3857"

artzaifman — Mon, 11 May 2020 20:08:05 GMT

Great answer and thank you.

Re: determine area of polygon in "EPSG:3857"

JoshuaBixby — Mon, 11 May 2020 20:27:44 GMT

If any of my responses answered your question, please mark one of them as the correct answer or mark the question as assumed answered to close it out.

Re: determine area of polygon in "EPSG:3857"

artzaifman — Mon, 11 May 2020 20:51:10 GMT

To close out this issue, sr=8858 works fantastically and here's the sq. meters of TX from "area" property:

692653163631.7217 (vs. 956858520477.5554 using sr=3857).

Re: determine area of polygon in "EPSG:3857"

JoshuaBixby — Mon, 11 May 2020 21:14:49 GMT

I am glad you found something that worked. In GeoNet system itself, a question is not closed out until it is marked "Assumed Answered" or one of the responses as marked as "Correct Answer".