I have statewide land ownership data and have created a mask to project ownership for only one county. I need to calculate the acreage of each owner within that county. Is there a quick way to achieve this? I'm using ArcGIS 10.
You can use the 'Calculate Geometry' (Right click on an field in the attribute table) tool to determine the area of each polygon and then use the 'Summary Statistics' (in the Analysis toolbox) with the owner name as a Case field and the area field set to sum will produce a table that shows the total area for every unique entry in the owner name field.
Limiting the results to one county can be accomplished through a variety of ways. The most straightforward one is to use the county layer to clip out all the shapes from the ownership polygon for just that polygon. You could also use the polygon shapefile as an analysis environment in the workspace of your document (provided you have access to a license of spatial analyst)
Create a field in the attribute table to store the acreages. Do a select by location query that intersects the parcels with your mask. In the attribute table right click on the name of your new field and use the Geometry Calculator. You can select acreage from there.
You'll need to specify a projection, hopefully your data is in UTM or State Plane.
This YouTube video can walk you through the process: