After further research, I discovered that the malfunctioning Snap Raster setting was actually caused by a bug in ArcGIS 10. The solution seems to be to turn off Background Geoprocessing (Geoprocessing\Geoprocessing Options... then uncheck Enable in the Background Processing section), and perform the clipping operation, with Snap Raster specified, in ArcCatalog.
There is a discussion on the ArcGIS forums here: http://forums.arcgis.com/threads/50808-Another-Snap-to-Raster-problem
Apparently the bug is fixed in ArcGIS SP 4. I've installed the service pack but haven't tested yet.
Interestingly (or rather, annoyingly), if I perform the clip to my area-of-interest polygon with the Snap Raster specified (Snap Raster is DEM) as per the solution in the forum post, above, the Snap Raster works but the Landsat band has one more column and one more row than the DEM raster, which was also clipped in ArcGIS to the area-of-interest polygon. This is annoying because for further analysis I need the two rasters to have exactly the same number of rows and columns. A simple workaround is to perform a calculation with the Con() function that creates a new raster with the same number of rows and columns as the DEM, but that has the values of the Landsat band.
I think that the original poster may want to calculate for each cell the height above the first stream cell that would be reached by water flowing from the cell. So the 'nearest stream' is calculated along the downslope flow path, not euclidian distance. The references for this Height Above Nearest Drainage (HAND) are:
Rennó, C. D., Nobre, A. D., Cuartas, L. A., Soares, J. V., Hodnett, M.
G., Tomasella, J. and Waterloo, M. J. (2008) HAND, a new terrain
descriptor using SRTM-DEM: Mapping terra-firme rainforest environments
in Amazonia. Remote Sensing of Environment 112, 3469-3481.
Nobre, A. D., Cuartas, L. A., Hodnett, M., Rennó, C. D., Rodrigues,
G., Silveira, A., Waterloo, M. and Saleska, S. (2011) Height Above the
Nearest Drainage - a hydrologically relevant new terrain model. J.
Hydrol. 404, 13-29.
My rather kludgy implementation was:
1) Create a flow direction raster and flow accumulation raster from the DEM. The help files can walk you through this.
2) Create a stream raster from a flow accumulation raster by setting a threshold value for what is considered a stream (in my case 250 m^2). Merge with a sink raster because Australia is full of hydrologically relevant closed depressions. This is the drainage raster so set all values to a HAND of 0.
3) Calculate a raster of height above the next downslope cell for the entire area (flow elevation difference). This will be used in lots of iterations but only needs to be calculated once. You need to store text files for each irregular neighbourhood direction, e.g. for flow direction 64:
DirCode64.txt -
3 3
0 1 0
0 0 0
0 0 0
The calculation is performed once for each determinate value in the flow direction raster (powers of 2). For the odd-ball values that are not powers of 2, I used the height above the minimum elevation cell in the surrounding 3x3 neighbourhood.
4) Calculate the height above drainage iterating out from the drainage lines as:
flow elevation difference + HAND of next downslope cell.
This is performed once for each flow direction, added to the HAND raster, then iterating until the number of null cells in the HAND raster stops changing. I included an escape if the number of iterations got too high and save the output periodically so I can restart if/when the process crashes. The saving seems to be a slow step so I don't do it every iteration.
Hope this is clear enough. I'm sure the code could be cleaned up but I stopped when it was working. Thanks for help from other threads that pointed me in the right direction. Here is my code:
import arcpy
from arcpy import env
from arcpy.sa import *
arcpy.CheckOutExtension("Spatial")
env.workspace = "C:/Data/GIS_Data/DEM"
# Starting HAND raster with 0 for streams/sinks
outHandRaster = Raster("hnd20strsnk")
# use below for restarting iterations
#outHandRaster = Raster("hand20gh2")
inElevRaster = Raster("dtm20m")
inFDirRaster = Raster("fdir20m")
lstDirection = [1,2,4,8,16,32,64,128]
# Count the number of null cells in the initial stream raster
nullOutRaster = Con(IsNull(outHandRaster),1)
nullOutRaster.save("handNull")
cursor = arcpy.da.SearchCursor("handNull","Count")
nullCount = cursor.next()[0] # Count of null cells in outHandRaster
print "nullCount = ", nullCount
nullDif = 1 # anything but 0
# Calculation of floweldif raster – contains the elevation difference
# between each cell and the cell in the downslope direction.
# This block only needs to be calculated once for the area
for idx in lstDirection:
focalMaskFile = "C:/Data/GIS_Data/DEM/FocalStatNeighbor/" + "DirCode" + str(idx) + ".txt"
outElDifRaster = Con(inFDirRaster == idx, inElevRaster - FocalStatistics(inElevRaster, NbrIrregular(focalMaskFile), "MINIMUM"))
else:
# Calculate values for indeterminate flow direction cells
outElDifRaster = Con(inFDirRaster,inElevRaster - FocalStatistics(inElevRaster, NbrRectangle(3,3), "MINIMUM"))
outElDifRaster.save("floweldif")
# Iterative calculation of HAND raster
inElDiffRaster = Raster("floweldif")
maxIter = 100
i = 0 # iteration counter limits number of loops for testing
while nullDif != 0:
for idx in lstDirection:
focalMaskFile = "C:/Data/GIS_Data/DEM/FocalStatNeighbor/" + "DirCode" + str(idx) + ".txt"
outHandRaster = Con(inFDirRaster == idx,Con(IsNull(outHandRaster),inElDiffRaster + FocalStatistics(outHandRaster, NbrIrregular(focalMaskFile), "MAXIMUM"),outHandRaster), outHandRaster)
else:
# Calculate values for indeterminate flow direction cells
outHandRaster = Con(IsNull(outHandRaster),inElDiffRaster + FocalStatistics(outHandRaster, NbrRectangle(3,3), "MINIMUM"),outHandRaster)
i += 1
print(str(i) + " iterations complete")
if i % 5 == 0:
outHandRaster.save("hand20")
nullOutRaster = Con(IsNull(outHandRaster),1)
nullOutRaster.save("handNull")
cursor = arcpy.da.SearchCursor("handNull","Count")
newCount = cursor.next()[0]
print "nullcount = ", nullCount, "newCount = ", newCount
nullDif = nullCount - newCount
nullCount = newCount
print "nullDif = ", nullDif
cursor.reset()
if i >= maxIter:
outHandRaster.save("hand20gh2") #restart file
break
Best Answer
When you reproject in QGIS, it is using something called gdalwarp under the hood. Try explicitly setting the
-te(target extent) and -tr (target resolution) options explicitly to exactly match the raster you want to align to. These values are implicit if unspecified. To coerce conformity, you may also need to use the-tap(target aligned pixels) option, but depending on the alignment of your comparison (unprojected) raster, you may also have to warp that one in the same projection but with-tapon (ie warp both using-tap).To do this, set up your warp command in the usual way and then click the yellow pencil button by the text box at the bottom of the dialog. You can now edit the gdalwarp command following the documentation.