Help: Issue with Geoprocessing Intersect area - some areas too small or zero when they should be larger

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08-27-2021 07:38 AM
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MarkDoehnert
New Contributor II

I am trying to determine the public school properties that intersect with stream Resource Protection Areas (RPAs) in my county and the surface area of any intersections. I am using ArcGis Pro 2.8.2 Windows 10 version. I have two files,  "Resource_Protection_Areas.shp"   and "Tax_Administration_s_Real_Estate_-_Property_Owner_Addresses.shp" files that I got from the GIS data https://www.fairfaxcounty.gov/maps/open-geospatial-data

I ran Geoprocessing Intersect and got a map view and a list under map.  Map results look good!  But I cannot figure out how to get the area of overlap or which columns in the output is this area results data, and some of the results do not seem to make sense.
 
Based on what I do see, some of the resulting area totals are either (a) a very small number, or (b) showing up as zero (0) when I can see on the map there is overlap.
 
Map number 0432 01  0001  I believe  has the most overlap visually, with an area that should be acres based on Google Maps and other analyses. But I cannot figure out how to get the area of overlap or which columns in the output is this area results data.  Like the far right column of results for Map number 0432 01  0001  is  Shape_Area but it has a value of "0.000014."  But clearly the mapped overlap area is larger than this - acres in overlap.
 
And for an elementary school (see screenshot)  Map #  0262 02 0006 is an example where I get 0.000 but the map seems to show more overlap than none.
 
To get the school properties, I used OWNER_FULL_NAME   =   SCHOOL BOARD OF FAIRFAX COUNTY  or  SCHOOL BOARD OF  FAIRFAX COUNTY or SCHOOL BOARD FAIRFAX COUNTY  - there are 260 school properties.
 
 
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MarkDoehnert
New Contributor II

I contacted ESRI support - what we did was add a Field and Calculate area to square feet (or I can use acres). When we did this, the results seemed more logical. They are not sure yet why the total area was showing such small numbers, because in the Project Options, square feet was specified, and showing, and the coordinate system checks were fine too.

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DavidPike
MVP Frequent Contributor

Are you missing some screenshots? I don't see map numbers you're referring to.

When it comes to small shape area, you may likely have data in a geographic coordinate system such as WGS84, with shape_area being in square decimal degrees. I'd check the properties of your data and either put everything in the same projected coordinate system e.g. NAD83 something something.

or use a tool such as https://pro.arcgis.com/en/pro-app/latest/tool-reference/data-management/calculate-geometry-attribute... 

MarkDoehnert
New Contributor II

Hello David,

I was just told the data is projected in the coordinate system NAD 1983 Virginia North, which file Properties confirm.  Angular unit per Properties for file are degrees.  Extent in files shows as "dd." Units for the Project show as square feet.

And I only put up one screenshot as an example - like Map number 0262 18 0007 shows in the map but if I am interpreting the results in the table I get, it is 0.000001  and Map number 0262 02 0006 I get 0.000000 but the map clearly shows some overlap.

Shouldn't ARCGIS Pro show the units involved, and make it clearer to the user what one is seeing? And especially when the Units for the Project show as square feet?

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DavidPike
MVP Frequent Contributor

No idea, still sounds like something is in a geographic coordinate system, but if you say the intersected shape is in NAD83 Virginia, then I guess its not. 

Although for the final point I will say that the coordinate system and units of the project should never be relied upon when dealing with geometry attributes of data within the project.

Did you try https://pro.arcgis.com/en/pro-app/latest/tool-reference/data-management/calculate-geometry-attribute...  

Without  screenshots of everything you're describing, including the coordinate system info of all the relevant datasets I struggle to get a feel for the issue, so can't really help any further sorry. 

MarkDoehnert
New Contributor II

I contacted ESRI support - what we did was add a Field and Calculate area to square feet (or I can use acres). When we did this, the results seemed more logical. They are not sure yet why the total area was showing such small numbers, because in the Project Options, square feet was specified, and showing, and the coordinate system checks were fine too.

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