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Shape areas calculated incorrectly in ArcGIS Online

AdrianWelsh

Here is a link to a web map:

https://arcg.is/1L9qfz

Take a look at this trapezoidal-type object:

The shape area listed in the popup is 2,159.10 (I guess square feet).

If I measure this with the measure tool, the area comes much closer to what it should be:

(here it is 13,535.5 Sq Feet - but the actual shape is 13,508.624 square feet (US) in ArcGIS Pro‌).

The only thing I can think is happening here has to deal with Coordinate Reference Systems‌ since in desktop, we are using a custom coordinate system due to the survey-grade work (also the data was imported from a CAD dgn drawing). I have performed repair geometry on these polygons.

All of the polygons in my AGOL web map are off by the same amount (roughly 6.256 or so).

How do I correct for this?

I tired looking up this article (Problem: ArcGIS Online does not display the shape area and shape length attributes of a feature layer ) and doing the task there, but my shapes already have geometry and are being calculated so this did not pan out. I welcome any suggestions!

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DaveOrlando

Ya it was bad timing for a lesson. We were having field crews use collector to gather data and office staff ordering supplies based on the lengths. Just when you think you get a win for GIS.......

AdrianWelsh

This is some good sleuthing, Dave. Do you have links for your map service and feature service?

I suppose this is one of the main reasons why you would want to use ArcGIS Enterprise to publish data versus just hosted on ArcGIS Online.

We are learning to have an attribute in the table that records our shape area and just post that instead of using the shape area field. As long as that is populated (and correctly), we should be good to go for decision making (ideally).

DaveOrlando

to answer my own question after some more experimenting.....

Yes, ArcOnline is treating their services different than our own ArcServer services.

a few things that got me to this conclusion are that when you open the attribute table in the ArcOnline map, you see all the 'proper' UTM lengths when they are outside the map extent.

try this by:

zoom to where there are no features (roads for me)
open attribute table and screen shot some of the IDs and Length
close the table, zoom to one of those features and re-open the table
the length will now change to their coordinate system.

also, you will notice all the generalization happening when you zoom from way-out to way-in which also supports the fact that they treat their services different than our own.

something to be very aware of when you make applications for the non-GISrs and critical decisions are being made specifically regarding the length of features......

DaveOrlando

finally had some time to do some reading and more experimenting and I have a follow-up question for anybody willing......

I've published the same service on my server and ArcOnline, both as UTMZ11 (wkid: 26911)
I have verified that both services state 26911 in their service definitions
both services show UTM length in ArcMap
my service shows UTM length in ArcOnline/Map popup
ArcOnline/service shows WebSphere coordinate system length in the ArcOnline/Map popup

My question is: how is it treating the ArcOnline Service different than my own if they are both published as 26911. Is ArcOnline actually detecting that this is 'their' service and taking the liberty of converting the length on the fly?

I understand what they are saying about reprojecting on the fly (esri response above) by why for their services and not ours, not that I want that but how are they being treated different?

Thanks

DanPatterson_Retired

It is true

The link again https://support.esri.com/en/technical-article/000011356

the web stuff distorts distance and direction. There is a link at the bottom of the link on how to choose an appropriate projection, which means if you are coding, you can implement a 'correction' within code using another projection

DaveOrlando

I find this disturbing and I'm not sure I believe a projection change would case such a drastic difference, but I could be wrong. I ran into this with Collector and WebMaps / AppBuilder. Strange tho that in collector it displayed the correct length in the popup header, (circled in pic) but wrong in the attributes, almost x2 (removed from this pic)

haven't had time to investigate much, just had to remove from popup,

AdrianWelsh

Haha, or maybe like Shape_Area_Times_Erroneous_Factor. Shape_Area_In_Biblical_Cubits perhaps?

DanPatterson_Retired

Can you set the alias for Shape_Area to Shape_Area_ish

AdrianWelsh

Long story short - it ain't happenin' in out-of-the-box ArcGIS Online.

I contacted tech support and after some back-and-forth, this kind of was the official answer:

ArcGIS Pro and ArcGIS Desktop calculate the measurements using the Spatial reference of the data. On the other hand, ArcGIS Online performs the measurements using the spatial reference of the basemap. In other words, your data not only gets reprojected on the fly but the area and length measurements will use the spatial reference of the basemap (WGS 1984 Web Mercator (auxiliary sphere). Furthermore, ArcGIS Desktop and ArcGIS Pro perform planar measurements while ArcGIS Online performs geodesic measurements.
Every map projection causes distortion of shapes, areas, directions, and/or distances. WGS 1984 Web Mercator and WGS 1984 Web Mercator (Auxiliary Sphere) use a conformal projection that preserves direction and the shape of data but distorts distance and area.
In order calculate the area and length of the features with reference to a specific spatial reference in ArcGIS Online, you will need to use a custom basemap with the specific projection. However, you will need to host that basemap on ArcGIS Server.

For my land surveyors who need to know at least general measurements, I will have to hide the shape area field and just include the actual calculated area into another field and display that. I'm disappointed by this but it is what it is...

AdrianWelsh

Right, I am not trusting Web Mercator to be accurate but I was hoping for it to be close, at least. I feel like there's gotta be a way to trick ArcGIS Online into calculating the shape area again, with things being reprojected on the fly. I wonder if this is an actual bug...

DanPatterson_Retired

I would do your checks in PRO. I suspect it is the web Mercator projection which wouldn't be the best to use for accurate planar measures.

AdrianWelsh

Haha, yeah I looked at the "obvious" issues, or at least I think I did. It's roughly 10.76 square feet in a square meter.

DanPatterson_Retired

you have ruled out the obvious square meters versus square feet thing (to lazy to get out the calculator)

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