Monday, July 4, 2011

Convert raster to vector polygons with Global Mapper

Global Mapper 12 has a function tuck away in a pop up menu to convert a grid layer or image layer into vector polygon features. It's the Create Area Features from Equal Values in Selected Layer accessible from the Overlay Control Center's right click pop up menu. Using this function is straightforward - simply load in a grid layer, fill in some parameters, and run the command.

To show how this works, see the example below in which a forest cover ArcGrid ASCII file is vectorized into a polygon vector layer at 10% cover interval.
  1. Start Global Mapper. Load in a grid layer e.g. Cov.asc

  2. Select ALT+C.

    The Overlay Control Center is displayed
  3. In the Overlay Control Center, mouse right click on the grid layer.

    A pop up menu appears.

  4. Choose Create Area Features from Equal Values in Selected Layer.

    The Setup Equal Value Area Creation dialog box appears.

  5. Optional. Change the Layer Description, Value Attribute Name, or Area Classification.
  6. In the Maximum Match Distance field, type in a value e.g. 0.05.

    Note: The example grid layer cell values are actually in %, even though the screenshot shows meters. It's just that Global Mapper assumes all grid layers are elevation related layers.
  7. Click OK.

    The vector polygons are generated.

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