= Comparison of lidar dems = We patch lidar data on to ASTER data during delivery creation and need to know how accurately that data has been patched and, if it is differing, to what level it differs. To do this we can use: {{{ demcompare.py -d lidarpatched.dem -n navfile.sol --lidar path/to/ascii -l projection }}} Unless you have an SSD available in your machine this will probably take a while. This will create a patched aster dem to compare to the lidar dem, this will not be smoothed and as such is the raw ASTER data. It will give 2 outputs on completion the first is only the area covered by the lidar files, the second is the entire dem. All outputs are in metres. {{{ Temp folder created at:/tmp/grassdb-24688-115643 Calculation performed:GB13_10_2014_156_lidar_ASTER_bng.dem-ASTER_tiles.dem The maps were compared in:WGS84LL GB13_10_2014_156_lidar_ASTER_bng.dem was imported in:UKBNG ASTER_tiles.dem sum :1420384.824150 GB13_10_2014_156_lidar_ASTER_bng.dem sum :1444242.484528 Comparison sum :-23857.657357 Difference statistics: Min: -69.939673828125 Max: 32.3908696899414 Sum: -23857.6575275853 Mean: -1.8243983732955 Median: -0.985404 Absolute mean: 5.99935342521819 Std deviation: 8.54745020068044 Total cells: 39672 Non-null cells: 13077 Unmasked statistics: Min: -69.939673828125 Max: 32.3908696899414 Sum: -24706.5998933041 Mean: -0.641529909984007 Median: 5.49316e-07 Absolute mean: 2.21956360209108 Std deviation: 5.18281446074588 Total cells: 39672 Non-null cells: 38512 }}} Preferably the mean (non-absolute) for the lidar area would be around 0 metres, but in practice it tends to be around 2 to 4 metres for UK data and varying for UTM data. You can compare 2 dems already created using {{{ demcompare.py -d dem1.dem -c dem2.dem --lidar /if/needed }}} Which will skip the patching process and just give the dem outputs, if you use identical dems the outputs will be 0. === Non standard outputs === There are a few other options for deeper analysis of dems, and analysis of trends over years {{{ --output OUTPUT output the resultant comparison DEM for inspection in another program. }}} Use this to inspect the created dem and comparison dem in envi, you can confirm figures from these. {{{ --histogram HISTOGRAM Create a histogram and output it as a png at specified location. Include file extension. --png PNG Create a PNG of the difference dem at the specified location. Include file extension. }}} The histogram option provides a better visualisation of the mean data. PNG can be used to provide quick looks at a heatmap of the data for comparison. === Batch processing === {{{ --csv Output as csv format for batch processing. --script Use with csv to indicate return rather than print }}} Use these two for batch processing, can be useful for creating profiles of whole years worth of data. These would be helpful if we wanted to see our overall accuracy over the year.