Important: the current version of QGIS was tested for GeoTiff format only. So we will use only TIFF format for the lab. There is a problem with image rendering on QGIS for Windows. App-tly, QGIS can render ONLY georeferenced images. When I tried to open an image (w/o georef) the system shows it in TOC, but there is no image on the map canvas. For the excercise we use amherstopo.tif image, not the JPEG. |
You need a scanner to scan your map. If it is a large map, you need a large (map) scanner. For projects, it will probably be better for you to try and work with maps that fit on an 8 x 11 sheet of paper.
Before you do the scan, make sure you note the coordinates of grid lines that will be in your scanned image. Refer to Appendix A “ Amherst Topo Map” as an example. It is critical that you include some grid “crosshairs” that you can use as ground control points.
Scan the map to a TIFF file (or better yet a jpg file if the scanner software will do it). Refer to Appendix B, to learn about different image formats.
To use the .tif(.jpg) file in a GIS we must first georeference it. If we brought it into a GIS packge (QGIS in our case) as it is, it would have a simple Cartesian plane coordinate system that is based on the number of pixels in the file. We want to assign the .tif (.jpg) file to the correct projection system that it represents, and thus change the coordinate system to represent this projection.
This involves (a) specifying the .tif (or .jpg’s) coordinate system in QGIS; (b) Digitizing specific control points; and (c) Rectifying or “stretching” the scanned image to fit the new projection.
For the remainder of this excercise, please use datasets available at Appendix C