With at least one of the themes open, choose View-Properties from the View menu
Set "Map Units" to the units of the map source's projection (for City of Austin data, this is feet - for other themes you must know what projection that was used - the projection parameters will specify the unit)
Set "Distance Units" to whatever unit you would like to use as a measuring unit (e.g., feet or miles)
You can use the measuring tool and see the results in the "Distance Units"
you specified.
Calculating Area, Perimeter and Length of
Shape Features
The easy way:
Load the great X-Tools extension. This was created by the Oregon
Department of Forests. What we have in Sutton Lab is the version that
automatically sets your view properties - map units to feet, which is good
when using the local Austin and Round Rock data sets. To load it, start ArcView,
then choose File-Extensions. Scroll down towards the end and you will
see the X-Tools extension. Check mark it and choose OK. Then
when you have a View on display, you will see the X-Tools menu item, which
includes a number of extremely useful capabilities including "Update Perimeter,
Area and Length". The results give you area, perimeter and length in feet,
plus area in acres (for polygons). If you need to add additional fields for
different units (e.g., hectares, square miles), see the
instructions below.
Important Note: You must
have write-access to the shape files to use this portion of the extension
because it is editing the shape file's attribute table. The data on the box-gis
server under the parmenter directory is read only, so you will have to copy
the shape file(s) out to your own directory first. Also, this extension won't
work if your original map units are not feet!
To download the X-Tools extension, go to the Oregon Department of Forests GIS web site. They have a additional version to use if your data is in meters (e.g., for UTM projections).
The harder way (but you learn more!):
Use the sample script that comes with ArcView 3.x called calcapl.ave. You
can find this script by going to ArcView's Help menu and choosing Table
of Contents - Sample Scripts and Extensions - Sample Scripts - Views - Data
conversion - Calculating geometry for shapes. You can copy this
script into your own project and attach it to a new tool on the interface.
To use this script you must have write access to the shape files, so copy
the shape files you will be using into your own directory before you use
this script.
Notes about using the Sample Script "calcapl.ave":
the third line of the calcapl.ave script is incorrect - correct it when you copy it into your own script as follows: "if (thePrj.IsNil) then" should read "if (thePrj.IsNull) then"
the script overwrites any existing area, perimeter and length fields. Occasionally you will find that a field was not large enough to hold the calculation (only asterisks appear in the field cell). If this happens, delete that field entirely, and re-run the script. This will replace the field with enough room to hold the calculations.
If you run the script from the script window, make sure the view window was your last active document, otherwise you will get an error. It's best to attach this script to an icon on your View menu - for a quick introduction to scripts and how to attach a script to a menu, go through the Avenue QuickStart tutorial in the Avenue manual.
Adding additional fields for different measurement
units:
Once you have these native map unit geometry values calculated (e.g., feet),
you can use the following method to create additional geometry fields in
different units (e.g., miles):
Make the relevant theme active
Open the theme's table
Choose Table-Start Editing
Choose Edit-Add Field - add a new field which will hold the area calculation (e.g., "length-miles") and have enough characters to contain the resulting calculation
Choose the Calculator tool (or Field-Calculate)
As the expression to calculate, type the expression to convert the Area, Perimeter or Length field to the new unit (e.g., if your original length was in feet and you want to convert it to miles, type [Length] / 5280 ) (See Peter Wallin's Conversion Table for conversion formulas for every unit of measurement imaginable, and then some)
The new calculation for each feature will be placed in this field. To save the results, choose Table-Save Edits
Choose Table-Stop Editing