Accurate orthophotograhy is dependent upon an accurate DEM. Orthos are typically rectified to a photogrametically compiled DEM. However, to save project costs, orthos can be rectified to existing DEMs such as those supplied by the USGS. The client should be aware of the differences between these DEMs. Typical accuracy of a USGS 30m DEM is ±33'. In contrast, in a compiled DEM with a 1:400 scale the accuracy is held to within ±4'.

Find out more about WAM's orthophoto products here.