How to read arbitrary pixel values from a geotiff image?

Hi

I'm looking for a function to read values from a geotiff image for an array of arbitrary lat,lon coordinates.

Something like this:

val = pixtiffread(filename,lat,lon,refmat)

imread for tiff images lets you specify a pixelreagion but I can't find a function that takes lat and lon vectors.

Any help would be appreciated. Regards Neil

Answer 1

[Image Analyst]

The help for imread says: "'PixelRegion': Cell array, {Rows, Cols}, specifying the boundaries of the region. Rows and Cols must be either two- or three-element vectors. If you specify two elements, the values denote the 1-based indices [start stop]. If you specify three elements, the values denote the 1-based indices [start increment stop], to allow image downsampling."

So why can't you use that? Just have x by the columns, and y be the rows? Or else why can't you just read in the whole image and extract the values like pixelValue = yourImage(y, x), where you do that for each x and y that you have?

[Neil Caithness]

The images are too large to load and then extract the x,y values - though of course that would be the simplest way. So I need some solution that reads the file and knows how to calculate appropriate offsets etc. but doesn't attempt to load the file into a matrix.

The {rows, cols} parameter for imread must be only two or three elements each specifying first and last rows and cols, as in a bounding box. I want to read pixel values for n-length x and y arrays.

Sorry if I've misunderstood.

[Image Analyst]

How big is your image? Can you sort your x and y points into where they would lie if you divided your image up into, say, 10 by 10 blocks? Then you can read in one block at a time, read pixel values for the coordinates that lie in that block, and do this for every block of the image.

[Neil Caithness]

Thanks, I will probably have to do it that way but I'm still hoping that someone might suggest an efficient solution for reading pixel values directly from geotiff files for arbitrary vectors of x,y coordinates.