Can I symbolize points in map units?

May 04 2011 | 1 comment
Categories: Symbology

I want to show point features with a specific size expressed in map unit (i.e. 660 ft) but I do not want to create a buffer layer to achieve the same effect. Is it possible symbolize points in map units?

You can do this in ArcGIS 10 using an expression and a size field in your attribute table.  To do this, you will use a formula that will convert your size in ground units (based on the map scale) to points (72 points to an one inch) which are the units used for symbols.

Here are two examples from which you can see the form of the general equation.

1. 1:24000 scale map; feature width in feet:

size in points = 1/24000 * width (ft.) * 12 in./ft. * 72 pt./in.

2. 1:50000 scale map; feature width in meters:

size in points = 1/50000 * width (m.) * 100 cm./m. * 1/2.54 in./cm. * 72 pt./in.

You can see that the general equation would be:

size in points = RF * ground width * ground width conversion to inches * 72 pt./in.

In your attribute table, you need to have the size attribute in ground units (e.g., feet as in your question). Then on the Symbology tab of the Layer Properties dialog, using the Features - Single Symbol renderer, click the Advanced drop down and select Size... Then click the drop down next to "Size Points by Value in this" and click the expression option. Click the Expression Builder icon to the right and in the Expression area, type in the expression. Click OK to all windows and your features will now be shown in ground units.

For only one symbol, you can use Proportional Sym posted by Aileen Buckley on May 7 2011 7:20AM
If you have only one symbol, you can actually do this with our Proportional Symbol renderer. If you have more than one, use the appraoch described in the original answer.

For the proportional symbol approach, here is how you do it:

Make sure that your size field in your data represent either the radius of the circle point symbol or the area of the feature. Then, suing the Quantities -> Proportional Symbol renderer, you can set the Value field to be your size attribute and in the drop down for Units, select the units used to represent your size attribute. Itâ€™s just as simple as that!