October 22, 2007

Coordinate system for shapefiles exported from GeoLytics CensusCD products

Those ACIC lab users who export a map from GeoLytics as a shapefile and then want to display these data with other GIS layers will need to know the coordinate system and--if applicable--the projection of the shapefile exported from the GeoLytics product so that this information can be assigned to the data in ArcGIS or ArcView 3.3. If this step is omitted in a GIS setting, the GeoLytics data will not display properly with data from other sources, nor will these shapefiles be capable of being projected or supporting distance or area calculations.

There is no good way to discern this information precisely (since GeoLytics' own documentation provides no metadata) but to ask the vendor directly. Katia, the data specialist in technical support, explained to me that the shapefiles that result from a data export are unprojected and are in the NAD83 (North American Datum of 1983) coordinate system. I tested this with several of our other layers on the I:\ drive and with some known distance measurements to confirm that this seems correct.

September 21, 2007

Clipping Raster Layers in ArcMap

Students can use the county land use map.mxd file in the H drive !!!!ACIC folder to create their map. They will need to disable the current clipped county to select their county and may need to manipulate the layout view based on the shape/size of their county.

The Orning Land Use data is available from the ACIC computers on the Mapfiles drive. See MNMapS\Data\LyrFiles and choose landuse.lyr.

ArcMap users may encounter problems when attempting to clip a raster layer to a certain extent. For example, you may be trying to show land use or demographic data for a county. If the data layer covers a wider geographic area, the usual procedure is to use the ArcToolbox "Clip" tool to show only data within a specified area. To do this you would open ArcToolbox and select the "Clip" tool under "Analysis tools" -> "extract". You would then input the features, the clipping pattern, and the output data. Unfortunately, when the data you wish to clip is a raster layer, it will not show up in the Clip tool's available layers.

To get around this problem, it is possible to clip the entire data frame. This will cause only data within a certain region to be displayed.

To clip the data frame, first select the area you want to use as a boundary. If you only want to show data within a county, you would use the Select Features tool (an arrow with a square next to it in the toolbar) to select the county you are interested in. Selected features are outlined in light blue. Next, right-click on the active data frame (a yellow icon usually on the left sided of the ArcMap program, it may be named "Layers"). Select "Properties" from the bottom of the drop-down menu. In the properties dialog, click on the "Data Frame" tab, and check the box marked "Enable" under the Clip to Shape section (about halfway down the window). Next, click the "Specify Shape" button, and select "Outline of Features". If you selected a county, you would choose the appropriate layer for county borders from the Layers drop-down list, and "Selected" from the Features drop-down list. Click OK on the clipping and frame properties windows, and your map should now display only the data inside the selected area.

You can also use this method to clip to shapes or boundaries drawn onto the map. For more details, go to ArcGIS Desktop Help (press the F1 key in ArcMap), and search for "Clipping data frames to a custom shape".