From the Rectification Control Points module in GTK+ GRASS the user can edit and manually enter control points.
Rectification Control Points
From this menu the user can:
- Turn control points on or off.
- Highlight the selected control point on the display.
- Check the Root Mean Square (RMS) error for the selected points.
- Manually enter known control points.
- Automatically project manually entered geographic coordinates to the users coordinate system (e.g. UTM).
This is a useful option for selecting geographic graticules on scanned maps.
The control points can be saved to a re-loadable XML points file. The XML file includes the image name, location and format, plus the complete list of control points. Using this format the image and control points can be easily re-loaded in a single step. The following is a sample control points XML file …
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A rectification Control Points module is added to the GTK+ GRASS Interface to make selecting and manipulating control points easier.
Image Rectification Window
The Control Points window opens in the GRASS Interface, as a special, split-pane, tabbed window. Users can select rectification control points for a GRASS raster file or an external image. Control points are saved to an XML file with optional output to a gdal_translate compatible file.
Features
The features for the Control Points Rectification module include:
- Control point selection for both GRASS rasters and external image files (TIFF, PNG, etc).
- Image file name and type, plus selected control points saved to a simple XML file.
- Control points selection from existing GRASS rasters and vectors or manually entered points.
- Control points manually entered from geographic coordinates or from the user’s projection coordinates (eg. UTM).
- Edit menu that displays RMS for selected points based on the chosen rectification order.
- Edit menu that lets users turn control points on or off.
- Predicted location for a selected point that displays in both panes to help users locate control points.
- Full zoom and pan functions for both panes in the rectification tab window.
Map images can easily and quickly be saved from the GTK+ GRASS GUI. Saved images include all currently selected objects in the display list.
The map is saved by default to a PNG image at the same size as the display canvas. Other image formats supported by GTK+ will be added.
A second image save option is available to save map images as a scaled PNG image. The user is prompted to set the display width in pixels, of the output image. This option can be used to output map images at the current GRASS resolution, which can in turn be encoded to GeoTiff files or GDAL compatible files.
Note: The scaled image output is dependent upon system memory.
