Extracting features from images

The ability to classify an image leads us to another remote-sensing capability. Now that you've worked with shapefiles over the last few chapters, have you ever wondered where they come from? Vector GIS data such as shapefiles are typically extracted from remotely-sensed images like the examples we've seen so far. Extraction normally involves an analyst clicking around each object in an image and drawing the feature to save it as data. But it is also possible with good remotely-sensed data and proper pre-processing to automatically extract features from an image.

For this example we'll take a subset of our Landsat 8 thermal image to isolate a group of barrier islands as seen in the following screenshot:

You can download this image here:

http://geospatialpython.googlecode.com/files/islands.zip

Our goal with this example is to automatically extract the three islands in the image as a shapefile. But before we can do that, we need to mask out any data we aren't...