Recently we had an E-mail from an Explorer user claiming that MrSID images did not seem to be supported. “Not so!” we maintained, but yet this user had no luck in opening his MrSID images which were being downloaded from a Web site. We took a closer look, and discovered that the projection had not yet been defined for his MrSID files.
The short story is that all data sources to be added to Explorer must have their projection defined, otherwise Explorer does not know how to project the data on-the-fly onto its globe.
But here’s the long story, and how we solved the problem, so you can too if you run into it.
They key indicator that a projection (spatial reference) has not been defined for a data source is this message:
It’s easy to (wrongly) think that the .sdw file of the SID contains the projection. In fact, it only contains the georeferencing, and projection information is written to an .aux file once set.
To set the projection, we started ArcCatalog, navigated to the MrSID file, and right clicked the file to open its properties. You can see that the project has yet to be defined.
We clicked Edit, then navigated through the options to find the projection definition, going from Projected Coordinate Systems > State Plane > NAD 1983 (feet), then chose the file below for FIPS 3200.
Once the projection was defined, we could add it directly to ArcGIS Explorer.
The good news for Explorer users is that defining the projection for data sources is something normally done by the GIS analysts that author and manage the data. In this case it was missing, but is always good practice to define.