Have you ever loaded two files into ArcGIS with different projections yet they still lined up? I am guessing all of you will say yes as this is one of ArcGIS’ best features, albeit it rather seamless and hidden. The automated feature that lines up these various spatial layers is called, projecting (or re-projecting) on the fly, and will be the focus of this month’s Geospatial Frequently Asked Question (G-FAQ). Similar to the last few months of the G-FAQ, I will start off with a short explanation of the topic followed by a tutorial video. What is projecting on the fly and how does it line up my spatial layers? When might projecting on the fly fail? Are there any special considerations to working with datasets that have been projected on the fly? In the distant past, ArcGIS did not have the ability to line up spatial layers, including vectors and rasters, that were loaded if their projections differed. When I say that spatial layers with different projections would not line up, here is an example. If you loaded a color raster image file with a WGS84 datum and geographic coordinates, then loaded a parcel layer from the same location with a WGS84 datum but UTM coordinates, the vector file would not appear over top the raster imagery. Rather, the two files would look to occupy different geographic spaces even though they are indeed from the same location. While I am not 100% sure when on the fly projections were added, they have been a part of the software since version 9.0 at least. So then, projecting data on the fly is a feature that is rather seamlessly done by ArcGIS where by the data frame of your current session assumes the projection of the first file you load. In the example above then, since we loaded the raster imagery first, the data frame would assume a geographic WGS84 projection. And then when the second file is loaded, it is automatically re-projected from UTM WGS84 to match the Geographic WGS84 projection of the data frame and the first spatial file. This on the fly projection continues as more and more spatial layers are loaded into the same ArcGIS session. It is important to note that files transformed on the fly are done so in memory only so that the original file remains unaffected. On the fly projections only work when each of the spatial layers has metadata defining the datum and coordinate system (i.e. its projection, see this past GFAQ for more details on this topic). Returning to the example above, if the first layer we loaded, i.e. the raster, was not projected (see the link above to determine if a dataset is projected), then the data frame would not assume any projection. And similarly, if the second spatial file we loaded was not projected but the first layer was, then on the fly projection would fail and the layers would not line up. While on the fly projections are a huge time saver, there are some points that need to be considered so as to avoid errors: Now that we have set the stage for on the fly projections, it is time to spend a few minutes on a visual review of this topic in the video tutorial you will find here. Source.