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I have a question regarding polygon data. I implemented "Calculate geometry" and the new column includes the area of interest to ha. The problem is with the accuracy of the data. The site area which is presented in the following image has different values from the derived area from ArcMap 10.5. In detail, the site area has a sum of 503645.235092 ha, and the area from "Calculate geometry" 528045.547832 ha. I used as a projected coordinate system the "ETRS89 / Albania TM 2010".

Why did this happen and is there another way to calculate the area?

Ideally, I would like those results to be the same.

enter image description here

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  • would you explain what is "derived area from ArcGIS" i.e. how did you get the siteArea field. You may check if the siteArea field exist before projecting the layer to make sure these differences are not because of different projections that have different spatial properties and distortions. Although Site Area field seems like rounded value of ProjectedSi some values are significantly different so it's very likely projection issue in my estimation. You may also calculate the area in field calculator using Python !shape.area! to compare.
    – Esmaeel
    Jun 16 at 20:45
  • The "derived area from ArcGIS" is the one found using the "Calculate Geometry" (in ha (=hectares). The siteArea field pre-existed at the attribute table. It is not calculated by me. Now I want to find the overlaps, so firstly I want to check the accuracy of the column "siteArea", in order to be sure that the process will be accurate. I calculated the area using Python !shape.area!, and the result is the same as the column of the "ProtectedSite" (the second column at the attached image). I can't figure out what is wrong with those data and what column to use at the next steps...
    – StGian
    Jun 17 at 7:22
  • @Σταυρούλα Γιαννακοπούλου as the Site Area field was already there and it's obviously not a dynamic field, there is a good chance that it was calculated at the time when the data were projected on a different coordinate, It's now irreproducible! and it has only 2 decimal digits so how much it can be trusted. I think by calculating geometry, verifying by field calculator and make sure about your projection, you are in a safe spot to trust ProtejectedSi and ignore siteArea field as it's not something you can produce again.
    – Esmaeel
    Jun 17 at 7:31
  • Thanks for your prompt response. So, given that I have a safe projection, in your point of view I should trust the result from the field calculator. Am I right?
    – StGian
    Jun 17 at 7:39
  • @StGain I'm saying if you can't reproduce SiteArea field and didn't know in what condition it was calculated, then you can't trust it. sosure, by verifying projection (for both dataset and the data frame), and matching field calculator and geometry results (they would produce different results if dataset and dataframe have different PCS), I would trust such result for sure.
    – Esmaeel
    Jun 17 at 8:01

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