# Calculating acreage using ArcGIS Field Calculator?

What is the expression to calculate an acreage in USA ? I am aware that there is a geometry calculate but when you convert raster to polygon it doesn't do it's job and I have to use the field calculator to manual it.

I am writing a model builder and in it I have tools for Add field and calculate them for 83 datasets.

• the automated "shape_area" field exists with geodatabase, but not with shapefiles. Also, the unit of the area is the same as the reference system of the data. – radouxju Jun 16 '14 at 14:38

In ModelBuilder, drop in the Calculate Field tool (under Data Management | Fields toolbox) and use the expression `!shape.area@acres!` (make sure Expression Type is `PYTHON` or `PYTHON_9.3`.)

Or, in ArcPy: `arcpy.CalculateField_management(YourFeature, Field, "!shape.area@acres!", "PYTHON", "")`

It's possible to use a wide range of units for both area and length calculations this way:

Shape and length properties of the geometry field can be modified with unit types expressed with an @ sign.

Areal unit of measure keywords: ACRES | ARES | HECTARES | SQUARECENTIMETERS | SQUAREDECIMETERS | SQUAREINCHES | SQUAREFEET | SQUAREKILOMETERS | SQUAREMETERS | SQUAREMILES | SQUAREMILLIMETERS | SQUAREYARDS | SQUAREMAPUNITS | UNKNOWN

Linear unit of measure keywords: CENTIMETERS | DECIMALDEGREES | DECIMETERS | FEET | INCHES | KILOMETERS | METERS | MILES | MILLIMETERS | NAUTICALMILES | POINTS | UNKNOWN | YARDS

You may include this python expression in your field calculator tool:

``````!shape.area@acres!
``````

http://resources.arcgis.com/en/help/main/10.1/index.html#//00170000004s000000

• Does anyone know the calculation to convert square feet to acres, supposing someone wanted to do this in PostGIS? – DPSSpatial Jun 16 '14 at 15:38

in the field calculator (CalculateField tool in model builder), you can use the following expression.

!shape.area@ACRES!

The actual formula or expression depends on your CRS measurement units, since acres is one unit and CRS units can vary - degrees, meters, feet, etc. Assuming you are in a projection, you'll probably have feet or meters.

1 acre = 43,560 square feet = 4046.85642 square meters

As radouxju mentions, shapefiles don't automatically track shape area, but feature classes in geodatabases do - only in the CRS units though. As the other answers suggest, using Field Calculator with a function is the only way to do this 'automatically' in ModelBuilder since the Calculate Geometry tool can't be accessed from there.

• when you convert raster or image to polygon, does it takes from the pixels from that raster or image ? Is there a way to smooth it a little bit ? How do you match the polygon with the image ? I am thinking it's not possible but.... – PROBERT Jun 17 '14 at 0:43
• @PROBERT The tool gives you a choice. If you smooth it, it of course won't match exactly - that's just the nature of the difference between a raster and a vector representation of data. – Chris W Jun 17 '14 at 1:05