A very useful add-in for ArcGIS Explorer Desktop users is the georeferencing add-in, written by Andriy Ivanov, a core member of the ArcGIS Explorer development team. The georeferencing add-in enables you to align a raster dataset without a spatial reference to its proper position on the map. The add-in uses a simple geometric (affine) transformation to create links between locations on the raster and their respective positions on the map.
This add-in will be particularly useful for for those wanting to add scanned maps, but can be used to position any raster lacking a spatial reference. Let’s take a closer look at how you can use this add-in.
Various raster types can be added directly to ArcGIS Explorer if they have a spatial reference. If they don’t, you’ll get an error message informing you that the coordinate system is not known.
One way to georeference the raster is using the georeferencing toolbar in ArcMap, which has comprehensive and powerful georeferencing tools and capabilities. But using this add-in you can perform simple georeferencing directly in ArcGIS Explorer.
We’ve chosen a sample raster, shown below. It’s a JPEG file captured from a portion of the World Streets basemaps around the Esri campus.
First, we’ll need to get the Georeferencing add-In from the ArcGIS Explorer Desktop Labs group on ArcGIS Online.
Once installed you’ll see the add-in on the ribbon in your Add-Ins tab.
Click to open it, and begin by choosing a raster you want to georeference. Before selecting your raster, you should zoom to the area roughly approximating where the raster should be located.
Once selected, the raster will appear in the middle of your map and also in your contents.
Now you can add control points – just click on the raster, then click on the basemap. The raster will automatically toggle off as you add control points (you can override this if you like), and by selecting the raster in your contents you can also adjust its transparency to make adding control points easier.
The yellow flag identifies the location in the original image, and the red flag points to its corresponding location on your map.
The add-in displays the coordinates of both points:
From here, just add control points to georeference your raster. You can also remove links if you’d like to redo them.
Three control points are required, and also the max you can add. Choose your points well – different corners will provide the best results. Once satisfied, click Georeference.
Here’s our newly georeferenced raster shown in its proper position on the map:
