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I have a numpy array that I want to convert to polygons using the gdal.Polygonize function, as outlined using this Polygonize a Raster Band approach.

Unlike the example listed, however, my data is currently in a 2D numpy array (my array in reality is not just random numbers, but has contiguous sets of values with the same number, and is appropriate for polygonization):

my_arr = np.random.rand((100, 100))
wkt = 'PROJCS["NAD83(CSRS) / UTM zone 10N",GEOGCS["NAD83(CSRS)",DATUM["NAD83_Canadian_Spatial_Reference_System",SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6140"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4617"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-123],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Easting",EAST],AXIS["Northing",NORTH],AUTHORITY["EPSG","3157"]]PROJCS["NAD83(NSRS2007) / UTM zone 10N",GEOGCS["NAD83(NSRS2007)",DATUM["NAD83_National_Spatial_Reference_System_2007",SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6759"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4759"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-123],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Easting",EAST],AXIS["Northing",NORTH],AUTHORITY["EPSG","3717"]]PROJCS["NAD83(HARN) / UTM zone 10N",GEOGCS["NAD83(HARN)",DATUM["NAD83_High_Accuracy_Reference_Network",SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6152"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4152"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-123],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Easting",EAST],AXIS["Northing",NORTH],AUTHORITY["EPSG","3740"]]PROJCS["NAD83(2011) / UTM zone 10N",GEOGCS["NAD83(2011)",DATUM["NAD83_National_Spatial_Reference_System_2011",SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],AUTHORITY["EPSG","1116"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","6318"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-123],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Easting",EAST],AXIS["Northing",NORTH],AUTHORITY["EPSG","6339"]]PROJCS["NAD27 / UTM zone 10N",GEOGCS["NAD27",DATUM["North_American_Datum_1927",SPHEROID["Clarke 1866",6378206.4,294.9786982138982,AUTHORITY["EPSG","7008"]],AUTHORITY["EPSG","6267"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4267"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-123],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Easting",EAST],AXIS["Northing",NORTH],AUTHORITY["EPSG","26710"]]PROJCS["NAD83 / UTM zone 10N",GEOGCS["NAD83",DATUM["North_American_Datum_1983",SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6269"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4269"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-123],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Easting",EAST],AXIS["Northing",NORTH],AUTHORITY["EPSG","26910"]]PROJCS["WGS 72 / UTM zone 10N",GEOGCS["WGS 72",DATUM["WGS_1972",SPHEROID["WGS 72",6378135,298.26,AUTHORITY["EPSG","7043"]],TOWGS84[0,0,4.5,0,0,0.554,0.2263],AUTHORITY["EPSG","6322"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4322"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-123],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Easting",EAST],AXIS["Northing",NORTH],AUTHORITY["EPSG","32210"]]PROJCS["WGS 72BE / UTM zone 10N",GEOGCS["WGS 72BE",DATUM["WGS_1972_Transit_Broadcast_Ephemeris",SPHEROID["WGS 72",6378135,298.26,AUTHORITY["EPSG","7043"]],TOWGS84[0,0,1.9,0,0,0.814,-0.38],AUTHORITY["EPSG","6324"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4324"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-123],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Easting",EAST],AXIS["Northing",NORTH],AUTHORITY["EPSG","32410"]]PROJCS["WGS 84 / UTM zone 10N",GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4326"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-123],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Easting",EAST],AXIS["Northing",NORTH],AUTHORITY["EPSG","32610"]]'
cell_size=1
min_x, max_y = 453193, 4736309

I can provide projection information via a wkt string (or any other representation) along with pixel size and position.

The issue is that the gdal.Polygonize function receives a gdal.Band object as its first argument. I have all of the necessary pieces to cook up a gdal.Band object (an array of numbers, coordinate reference, cell size), but I cannot figure out how to construct one with the Python API.

How would I go about constructing the gdal.Band object from the information I have available.

2
  • First specify your driver, then with your driver create a dataset with one band then write your array to the band, close to flush writes, and open the band; optionally you can then delete the dataset. This might help gis.stackexchange.com/questions/264793/… (not the crop part, the bit in the question which shows how to create a raster in memory). Apr 16, 2018 at 22:35
  • There is a post gis.stackexchange.com/questions/37238/… which has some options for you. The writing of an Esri ASCII file from the numpy array is novel but I would only use AAIGRID rasters for very small row/col rasters, less than 10k by 10k pixels, as they can become stubborn and clunky in larger sizes (not a technical term but a description of their performance). The 2nd answer by DavidF I believe is your best solution, it wouldn't take much to create the raster instead of opening it.. Apr 17, 2018 at 0:39

1 Answer 1

1

I just used this for gdal.Sieve which also needs a gdal.Band object so I leave my code here in case anyone needs.

driver = gdal.GetDriverByName("GTiff")
outdata = driver.Create('{}'.format("data/raster.tif"), num_row, num_col, 1, 
                        gdal.GDT_Float32, options=['COMPRESS=DEFLATE'])
outdata.GetRasterBand(1).WriteArray(my_array)
outdata.SetGeoTransform(geotransform)
srs = osr.SpatialReference()
srs.ImportFromWkt(wkt)
outdata.SetProjection(srs.ExportToWkt())

# Sieve
print('outdata', outdata.GetRasterBand(1))  # gdal.sieve accepts band object, not array
gdal.SieveFilter(srcBand=outdata.GetRasterBand(1), maskBand=None, 
                 dstBand=outdata.GetRasterBand(1), threshold=2, connectedness=8) 

outdata.FlushCache()  # writing the raster to disk

Just use polygonize on the place of my sieve.

0

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