Hey there, this is my first time posting in the forums and I have a bit of a tricky question. Hopefully somebody will be up to the challenge of answering it (or at least providing some insight).
I'm currently working on a research project attempting to detect changes in elevation between Digital Surface Models. My base imagery is a LiDAR scan in .asc format with accompanying orthoimagery. The LiDAR is projected in Transverse Mercator from GCS MGI in the Bessel 1841 spheroid and D_MGI datum.
My "Time 2" imagery is a DSM (in .tif) and accompanying orthomosaic (in .tif) and 3D point cloud (in .txt). These are projected in Transverse Mercator from GCS WGS84 with WGS84 spheroid and datum.
I can reproject co-register the imagery (using Erdas Imagine) no problem. The problem is that due to the nature of the data, when reprojected, only horizontal data is reprojected. The vertical data (stored as pixel values) is unaffected. This comes as no surprise. So while my two datasets can line up perfectly in the horizontal plane, my WGS84 DSM is, on average, about 50 meters higher than the MGI DSM (LiDAR).
My question is: how can I reproject, or modify, my raster based DSMs so that the elevation values are consistent? Differencing the two rasters to detect elevation change would be pointless when they are in different Geographic Coordinate Systems.
I also have a set of GPS points I collected in the area with a RTK rover unit. The points were collected in WGS84 and exported in both WGS84 and MGI formats. When compared against their locations in both rasters, their elevation readings are accurate between 20cm and 1m. I need to difference these two rasters and show the viability of the elevation change detection, but am stuck at this juncture. Does anyone have any insight as to how I can solve this problem? Thank you for reading and thinking.