Working with Length in GIS Attributes

Length Attribute Field…or Not

When you open a feature attribute table for line data in ArcGIS for Deskop, you might see a LENGTH field and you might not.  Why the inconsistency?  It has to do with the format of your feature class.  Geodatabase line feature classes include a system defined attribute field that stores the length of each feature; shapefiles do not.

But My Shapefile DOES Have a Length Field

Some shapefiles do have a length field, it’s true.  But they are not system- defined attribute fields.  System-defined attribute fields are ones that the software creates and manages.  You can read the values, but you cannot edit them.

If your line shapefile includes a length field, it’s because someone added it, either manually or by using some third-party tools.  It is a user-defined attribute field.  The lengths can be calculated and added to the attribute table, but there is no dynamic link between the features and length attribute value.  That means that if you edit a line feature and change its length, the length attribute value will not be recalculated automatically as it is with the system-defined attribute field.

What Units Are the Lengths In?

The geodatabase system-defined length attribute field displays lengths in the linear unit of the coordinate system of the feature class.  If, for example, your line feature class is in UTM coordinates, then the length values will be in meters because meter is the linear unit of the UTM system.

Can I Change the Units?

Yes, you can change the units of the system-defined length values.  Well, sort of.  You can add a new attribute field and use it to store length values that are based on the system-defined length values.

I’ll show you how in this video.

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