SRTM Image Processing Page 7

SRTM_Mask

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There is one final procedure to take prior to saving the file.

The boundary we have worked so hard to make needs to contract by one pixel. This will further reduce the noise produced by merging the mask with the elevation image. It will not just reduce it, but reduce it by several orders of magnitude, further reducing the amount of cleanup work you need to do after the mask is applied to the elevation file. Believe it or not there is a lot more error in the boundary between shore and ocean then you might think. And the fact that we have to contract the boarder by one pixel is actually a BIG argument for extra caution on the side of water and less caution on the side of land. So extending the land boundary out into the ocean is safer than accidentally extending the ocean two far inland.

Switch to the "Magic Wand Tool" we have already learned to use, and check again to make sure "Tolerance" is "1", "Contiguous" is selected. Then click once over the black area. Make sure you get all the major land areas, or any major islands floating out in the bay or out in the ocean.

Then navigate the menu control "Select->Modify-Contract".

You will get the following popup dialog box.

Put "1" in the "Contract By:" field, and click "OK"

Again navigate the menu control "Select->Inverse" or simply press "Shift+Ctrl+I" on the keyboard to invert the selection from land to ocean.

Now you will have selected all the ocean...but inland by just one pixel.

Check again to make sure the secondary color in the floating toolbar is pure white, not pure black.

Press "Del" on your keyboard. The shoreline should shrink by one pixel. If the entire ocean area is replaced by black, then you didn't do what I just said in the line above. -> Check again to make sure the secondary color in the floating toolbar is pure white, not pure black.

If you don't know how to do this, return to the section that tells you how to select primary and secondary colors

Finally before saving, we need to switch the representation of the image from 16 Bits per Channel to 8 Bits per Channel.

Do this by navigating the menu control "Image->Mode->8 Bits/Channel"

This will enable us to save it to a windows bmp file.

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