topic Re: Big Discrepancy in Acres Calculation in Data Management Questions

Big Discrepancy in Acres Calculation

Lubellski — Sat, 20 Mar 2021 03:39:40 GMT

Good evening, I have been delivered polygon data in 2 different projected coordinate systems. I've noticed some errors which I assumed to be due to using different projections, so I have been re-projecting everything to be standard. I would have chosen neither, but it is what it is.

In the attached file, there is a small area shown, in two different projections as follows:

Rose: WGS_1984_Web_Mercator_Auxiliary_Sphere, 1.42 Acres.

Hatched: NAD_1983_HARN_StatePlane_Oregon_North_FIPS_3601_Feet_Intl, 0.7 Acres.

The difference in acres calculation is DOUBLE but as you can see the area on the "ground" is definitely nowhere near double. I believe that the NAD83 value is more accurate.

Can someone help me explain this and a solution to it? Thanks very much, in advance.

Re: Big Discrepancy in Acres Calculation

DanPatterson — Sat, 20 Mar 2021 05:07:46 GMT

web mercator should never be used for area or distance calculations

FAQ: Why are my map, distance and area measurements wrong when using WGS 1984 Web Mercator? (esri.com)

moved the question to Data Management Questions since the Community Feedback is about feedback to the Community structure and functionality

Re: Big Discrepancy in Acres Calculation

Lubellski — Sat, 20 Mar 2021 21:36:25 GMT

Sorry for posting in the wrong place, this was my first post. And thanks for the link.

Appreciate it.

Eric

Re: Big Discrepancy in Acres Calculation

DanPatterson — Sat, 20 Mar 2021 23:57:47 GMT

no problem... the sad projection that comes with minimal warnings and apps that allow you to do things that they shouldn't with data in that projection.... that is the bigger problem

Re: Big Discrepancy in Acres Calculation

MarkScott — Tue, 23 Mar 2021 13:40:51 GMT

Hi Lubellski,

Web Mercator is a lousy map projection for area calculation. What is good at, is being convenient for tile storage. I think this article does a good job talking through it:

https://www.esri.com/arcgis-blog/products/arcgis-enterprise/mapping/measuring-distances-and-areas-when-your-map-uses-the-mercator-projection/

State Plane systems, being designed for a small area, try to minimize distortion, so you are definitely better off performing your area calcs there. Dan is right, the software and SAAS solutions let you do anything you want, even if its a bad idea!

Mark

Re: Big Discrepancy in Acres Calculation

Marokapara — Mon, 24 Jan 2022 08:32:21 GMT

@DanPatterson I've read this post and another one you commented in about area calculations.

I am not an expert in projects or coordinate systems. That's probably why I have this question.

I created a layer in AGOL using the imagery hybrid as my basemap. It has auto-calculated the Shape_Area as it usually does. I then measured the polygon using the measuring tool, and the areas are way out. I'm not sure what coordinate system the basemap is, but going through all the options of measurements in the tool, I can't get one to match it.

Has anyone else had this problem? It's probably a stupid question, but I can't figure it out...

It's affecting my Ha calculation too. Ideally, I'd like the measurements in square meters and hectares.

Re: Big Discrepancy in Acres Calculation

DanPatterson — Mon, 24 Jan 2022 13:40:01 GMT

Web Mercator coordinate system probably. Seems that it surely isn't a sq ft vs m^2 thing

You shouldn't be doing area measurements unless you can set the coordinate system to be used to a better projected one or use geodesic calculations

Geodesic versus planar distance—ArcGIS Pro | Documentation

discusses distance, hence, extend that to area and it gets worse. And the statement at the bottom of the page

To understand the specific differences in distance measurements between geodesic and planar, use the Measure tool to measure the distance between two locations in Planar mode and in Geodesic mode.

Re: Big Discrepancy in Acres Calculation

Marokapara — Mon, 24 Jan 2022 19:30:12 GMT

@DanPatterson So if I want to output the area in AGOL in hectares on the fly via an arcade expression, there's not an accurate way to do it.. unless I change the basemap project back a non-web mercator coordinate system?

Re: Big Discrepancy in Acres Calculation

DanPatterson — Mon, 24 Jan 2022 21:51:55 GMT

I don't do AGOL,

I would be surprised that Arcade doesn't have a means to provide the area in a better projection, but you should read up on geometry functionality if you need to use Arcade

Geometry Functions | ArcGIS Arcade | ArcGIS Developer

Found this as well

FAQ: Is it possible to calculate area in ArcGIS Online? (esri.com)