Calculate area of raster data in polygons

[PhilC_]

This appears to be a seemingly easy calculation. But I am having problems.  

Question: What is the percentage of a certain land cover class in each census block?

Data:
1) Census polygon
2) Land cover raster

Method: Tabulate Area tool - with census polygon as my zone and land cover as the class of interest.

Problem:  My output gives me areas much BIGGER than my census polygon (i.e. Percent land cover class is often greater than 100%...sometimes in the 1000s%).  These numbers are obviously incorrect, as area of vegetation can't be greater than the area of the census block (zone).  Both have linear units of meters and are projected.

What am I doing incorrectly?  Are there better ways to calculate area of raster in polygons?

Thanks in advance,

PC

[DuncanHornby]

Philip,

What size are your pixels with respect to your zonal data? Are you trying to compare for example a 1Km grid with a polygon that are a few hundred meters wide? I'm guessing some sort of re-sampling error is occurring. Ideally you want to ensure that your zonal data has the same cell size as your raster. May be you should create a raster version of this first and make sure its correct. You'll also want to ensure the snap to raster environment setting has been set.

Duncan

[PhilC_]

Philip,

What size are your pixels with respect to your zonal data? Are you trying to compare for example a 1Km grid with a polygon that are a few hundred meters wide? I'm guessing some sort of re-sampling error is occurring. Ideally you want to ensure that your zonal data has the same cell size as your raster. May be you should create a raster version of this first and make sure its correct. You'll also want to ensure the snap to raster environment setting has been set.

Duncan

Thanks Duncan.  The raster Land Cover dataset has a 250 m grid.  The polygon zones (census blocks) vary wildly, from less than 1 m2 to 30,000,000,000 m2.    I'll try creating a raster and report back.  

Thanks again for any help,

PC