Spatial Analysis Tip Sheet
Barbara Parmenter, PhD
1. Requirements for setting up for spatial analysis in ArcGIS
1.1. All data and the data frame must be in the same coordinate system:
Before doing any analysis functions in ArcGIS, make sure that all the data layers you will be using are in the same map projection/coordinate system, and that the data frame is also in that coordinate system. For ANY analysis in ArcGIS, the layers you are using in your analysis MUST be in the same coordinate system, and this must match the coordinate system of the data frame. If you don't do this, you'll either get errors, or it will appear to run, but nothing will happen. If your data is in a different coordinate system, the easiest way to convert it is to set the data frame to the correct one (e.g., Mass State Plane, Mainland, NAD 83, meters), then right-click on the layer that is not in that coordinate system and choose Data-Export. In the dialog box, say you want the new file to have the same as the data frame's coordinate system, and proceed from there. This creates a copy of the original shape file in the correct coordinate system. See tip sheets concerning projections if you need more help.
1.2. Set the Geoprocessing Environment Settings
For most or all of these analysis functions, it will also help if you set the Environment Settings. I recommend you set these before you do anything by going to the main toolbar in ArcGIS and clicking on Tools - Options, then clicking on the Geoprocessing tab, and finally clicking on the "Environments" button there. The critical things for you to set are:
Under the General Settings area:
Set the current workspace to be where you are pulling data from (just click on a folder, not a file - the folder within which most of your GIS data is located).
Set the scratch workspace to a temp folder on your local computer. Create a temp folder on the local computer under your user name if you have to.
Most important , set the Output Coordinate System to be the one you want all your analysis to be located in.
Set the Output Extent to be the largest extent of your study area – use an existing file that covers the study area or create a new one. This is critical for raster analysis – you must use the same extent for all your raster work. The Extent will always be rectangular even if you use an irregular polygon area to set it.
If you will be doing raster-based spatial analysis, it is good to set the Snap Raster to an existing raster data layer that has the extent and cell size you want for your analysis. The extent that you set will then be snapped to this raster to ensure that cells align properly.
Under the Raster Analysis Settings area:
Set the cell size (use this for all your raster analysis, so think about the cell size carefully).
Set a mask if you want (a polygon layer that represents the study area – the area outside this layer will not be included in the analysis.
If you use an irregular polygon layer to set this, the mask will reflect that.