Geo-referencing is the process of taking a digital image and adding geographic information to it so that GIS software can ‘place’ the image in its appropriate real world location. Let’s take for example the same Survey of India map we used in module 1E – sheet 52d/6. First step is to convert the PDF format to a regular JPG file through any website, e.g. ilovepdf.com. See image 1 below
Next, in QGIS select “Georeferencer” from the “Raster” menu and click Open raster (blue tile) icon to load the JPG file. Next click on each of the 4 corners of the map (excluding the boundary) and enter the corresponding latitude and longitude mentioned on the map. Important: enter the lat / lon in decimal degrees (e.g. 76 deg 15 min corresponds to 76.25 decimal degrees as there are 60 minutes in one degree). Ensure to set the CRS as “EPSG:4326 – WGS 84”. See image 2 below.
Once done for all 4 corners, click the georeferencer icon (green triangle) and set the default Transformation type and Target SRS. Click OK. You ll have to click the georeferencer icon once again to do the actual georeferencing. After this a new raster layer will be added to QGIS showing the Survey map in its appropriate location on the basemap. See Image 3+4 below
You can zoom in and hide/show the new SOI map layer to verify the matching location of streams, settlements, roads, etc. on the OSM basemap.
Digitizing map features
Once geo-referenced we can digitize individual elements from a map in QGIS. Settlements, roads, trails, rivers, passes, peaks, lakes, etc. For this we first create a new vector layer in QGIS – select “Layer”, “Create Layer”, “Shapefile Layer”. Provide a file name and geometry type (1. points: passes, peaks … 2. linestring: trails, streams… 3. polygon: lakes, forests…). In this case we chose LineString. See Image 1 below.
Ensure the “Digitizing Toolbar” is enabled in “View”, “Toolbars” menu. Next select the new layer and click the “Toggle Editing” icon on the toolbar to start digitizing trails (= adding lines to new shapefile layer). Choose the “Add Line Feature” icon on the toolbar to start digitizing. See Image 2 below.
Zoom into the map near Tundah village (bottom right corner of map) and click on the individual line segments marking the trail towards the Jhundhla jot pass. Once complete, right click and click “OK” to store your first digitized trail in the new layer. You can now hide the Survey map raster layer and directly view your digitized trail (in the shapefile vector layer) on the OSM basemap or Google Satellite map. See Image 3 + 4 below.
In both cases we can see that the actual trail on the ground (as per OSM or Satellite map) slightly differs from the digitized trail from the Survey map. This is acceptable as Survey maps are manually carto-graphed maps, created more than a century ago without modern day satellite technology. Overall the accuracy is pretty good to be useful as a reference for exploration.
Exporting data
After digitizing features from a geo-referenced map we can export the same through QGIS so they can be used for navigation in the mountains. Right click on the “Survey trails” shapefile layer and choose “Export”, “Save Vector Layer as…”, format “GPX eXchange Format”, provide a name and uncheck the “id” attribute. This will export your digitized trail in GPX format which can be read in most other mapping apps. See Image 1 below.
Send the GPX file to your phone and open in OSM. You are now ready to explore this Survey trail offline! See Image 2 below. Except for one diversion in the initial section, you can see that the trail roughly matches the hiking route in OSM.
Assignment
Submit the form below acknowledging the fact that you have gone through the above tutorial.