Extract Point From Raster in GDAL

Question

I have a raster file and a WGS84 lat/lon point.

I would like to know what value in the raster corresponds with the point.

My feeling is that I should use GetSpatialRef() on the raster object or one of its bands and then apply a ogr.osr.CoordinateTransformation() to the point to map it to the raster's space.

My hope would then be that I could simply ask the rasters' bands what is at that point.

However, the raster object doesn't seem to have a GetSpatialRef() or a way to access a geo-located point, so I'm somewhat at a loss for how to do this.

Any thoughts?

Answer 1

Say i have a geotiff file test.tif. Then followin code should look up value somewhere near the pixel. I am not that confident for the part looking up cell, and will fix there is error. This page should help, "GDAL Data Model"

Also, you may go to gis.stackexchange.com to find experts, if you haven't.

import gdal, osr

class looker(object):
    """let you look up pixel value"""

    def __init__(self, tifname='test.tif'):
       """Give name of tif file (or other raster data?)"""

        # open the raster and its spatial reference
        self.ds = gdal.Open(tifname)
        srRaster = osr.SpatialReference(self.ds.GetProjection())

        # get the WGS84 spatial reference
        srPoint = osr.SpatialReference()
        srPoint.ImportFromEPSG(4326) # WGS84

        # coordinate transformation
        self.ct = osr.CoordinateTransformation(srPoint, srRaster)

        # geotranformation and its inverse
        gt = self.ds.GetGeoTransform()
        dev = (gt[1]*gt[5] - gt[2]*gt[4])
        gtinv = ( gt[0] , gt[5]/dev, -gt[2]/dev, 
                gt[3], -gt[4]/dev, gt[1]/dev)
        self.gt = gt
        self.gtinv = gtinv

        # band as array
        b = self.ds.GetRasterBand(1)
        self.arr = b.ReadAsArray()

    def lookup(self, lon, lat):
        """look up value at lon, lat"""

        # get coordinate of the raster
        xgeo,ygeo,zgeo = self.ct.TransformPoint(lon, lat, 0)

        # convert it to pixel/line on band
        u = xgeo - self.gtinv[0]
        v = ygeo - self.gtinv[3]
        # FIXME this int() is probably bad idea, there should be 
        # half cell size thing needed
        xpix =  int(self.gtinv[1] * u + self.gtinv[2] * v)
        ylin = int(self.gtinv[4] * u + self.gtinv[5] * v)

        # look the value up
        return self.arr[ylin,xpix]

# test
l = looker('test.tif')
lon,lat = -100,30
print l.lookup(lon,lat)

lat,lon =28.816944, -96.993333
print l.lookup(lon,lat)

Answer 2

Yes, the API isn't consistent. The raster (the data source) has a GetProjection() method instead (which returns WKT).

Here is a function that does what you want (drawn from here):

def extract_point_from_raster(point, data_source, band_number=1):
    """Return floating-point value that corresponds to given point."""

    # Convert point co-ordinates so that they are in same projection as raster
    point_sr = point.GetSpatialReference()
    raster_sr = osr.SpatialReference()
    raster_sr.ImportFromWkt(data_source.GetProjection())
    transform = osr.CoordinateTransformation(point_sr, raster_sr)
    point.Transform(transform)

    # Convert geographic co-ordinates to pixel co-ordinates
    x, y = point.GetX(), point.GetY()
    forward_transform = Affine.from_gdal(*data_source.GetGeoTransform())
    reverse_transform = ~forward_transform
    px, py = reverse_transform * (x, y)
    px, py = int(px + 0.5), int(py + 0.5)

    # Extract pixel value
    band = data_source.GetRasterBand(band_number)
    structval = band.ReadRaster(px, py, 1, 1, buf_type=gdal.GDT_Float32)
    result = struct.unpack('f', structval)[0]
    if result == band.GetNoDataValue():
        result = float('nan')
    return result

Its documentation is as follows (drawn from here):

spatial.extract_point_from_raster(point, data_source, band_number=1)

data_source is a GDAL raster, and point is an OGR point object. The function returns the value of the pixel of the specified band of data_source that is nearest to point.

point and data_source need not be in the same reference system, but they must both have an appropriate spatial reference defined.

If the point does not fall in the raster, RuntimeError is raised.

Answer 3

project = self.ds.GetProjection()
srPoint = osr.SpatialReference(wkt=project)

done... with that, the vector file has adopted the projection from input raster file