If you use the GIS Hub's Web Map Services (WMS) and/or Web Coverage Services (WCS) you may be wondering what's up with the layer IDs in the WMS and WCS services changing from time to time. If you have an application that references one of those layer IDs and if you don't change the layer ID when notified via the NDGIS listserv or the NDGIS Twitter feed, you may find that the layer is not what you expected.
For example, layer ID 25 currently references the Bismarck-Mandan 2009 imagery. In the future, the GIS Hub configuration may be changed so that the Bismarck-Mandan 2009 imagery uses layer ID 24 and the Bismarck-Mandan 2005 imagery uses ID 25. If you don't make the corresponding change in your application, you will be viewing the Bismarck-Mandan 2005 imagery. So why does this happen? This didn't happen in the past did it?
As Esri moved from the older ArcIMS technology to the newer ArcGIS Server technology, a number of good features failed to make the transition. The static layer ID was one of them. In ArcIMS the layer IDs were set by the GIS Hub administrator and would remain named as specified, even if other layers were added or deleted. This is not the case in ArcGIS Server. The software sequentially assigns the layers their unique IDs but they are dynamic. Any layers addedd or deleted will reset the layer IDs. Hopefully this "feature" will be fixed in a future release of ArcGIS Server.