Part of the Grassroots Mapping Curriculum series.
Creating a map from a set of aerial photos
MapKnitter (MapKnitter.org) is a free and open source tool for combining and positioning images (often from MapMill.org) in geographic space into a composite image map. Known as “orthorectification” or “georectification” to geographers, this step covers the process of figuring out where images can be placed on an existing map, and how they can be combined, or “stitched” together. You are likely to have many images of overlapping or identical areas, which is why MapMill or some type of sorting is used to determine which source images to use from the original set.
MapKnitter can make maps from any image source, but it particularly lends itself to making maps with balloons and kites. The manual process of making maps with MapKnitter differs greatly from automated aerial imaging systems. In those systems the imaging is of a higher precision and processed with spatial and telemetry data collected along with the imagery, typically at higher altitudes and with consistent image overlap in the flight path sequence.
With MapKnitter the cartographer dynamically places each image and selects which images to include in the mosaic. Although the approaches are similar in that they use some type of additional information (usually pre-existing imagery of a lower resolution) as a reference, and that they are bound to specific cartographic elements such as map scale and map projection.
Learn more about MapKnitter, including advanced techniques, in this list of tutorials:
- Introduction to MapKnitter - Learn to turn aerial images into maps with the Public laboratory's open source MapKnitter.org tool. Upload an image, rotate, distort and stretch it onto a reference map, and use the transparency and outline modes to check how close the fit is. Then export to GeoTiff or OpenLayers/TMS.
- Exporting maps with MapKnitter - Learn how to export your completed maps in GeoTiff, TMS/OpenLayers, and JPG formats, and how to tune your export resolution to optimize for size. Learn to use advanced options to see the range of possible resolutions, and avoid exporting at excessively high resolutions.
- Decisions with distortions - Learn about the design process of stitching maps when images are highly distorted or include tall buildings. This tutorial is in Photoshop, but the techniques are applicable to MapKnitter as well.
- Infrared multispectral compositing (no video, in progress) Learn how to combine an infrared dataset with your map to assess photosynthesis. (Also: learn how to collect infrared imagery)
- Photoshop tutorials:
- Choosing a resolution
- Importing a base layer from google maps or another source
- Warping a difficult image with the mesh "warp" tool
- Turning a Photoshop-produced map into a GeoTiff using MapKnitter