One of the things that make a cartographer is the ability to use image tiles of the ground to overlay the maps.
Cartoon maps are fun and colorful:
But these days, it’s also much cooler if you can put all these elements and superimpose them on a real satellite image on the ground.
Matching the database file and location of these shapes to a real life satellite image is called georeferencing.
And it’s fairly easy if you already have a reference layer on hand.
Here are the steps in georeferencing an image. For this exercise, I used NCR roads as my primary reference.
Step 1: Add your reference layers and add the image that you want to georeference.
Step 2: Activate the Georeferencing toolbar after enabling editing of layers. I also opened the image using Windows Photo Viewer so that I can examine the elements of the image that I can match later.
Step 3: Go to effects toolbar for transparency.Transparency will make the color (in this example, baby pink color on map) lighter so that you can see the image underneath.
Step 4: Adjust transparency levels.
Step 5: Play matchmaker to the prominent points in the image and in your reference layer by adding control points. Of big help to me is the shape of UP Diliman and Quezon City Circle. The blue lines indicate my chosen control points. Just click first the “Add Control Points” button on the toolbar. To create one point, just click the point on the image then click the corresponding point on the reference layer. After this, click adjust or just check auto-adjust button on the Georeferencing toolbar.
Step 6: Observe the results after adjustment. As you can see, mine’s location is pretty defined by the time I adjusted.
Step 7: Check the RMS (root mean square) errors. Delete points which have high errors. The lower the better. You can also add more control points and replace the ones you deleted. In case you warped your image, you can just click “Reset Transformation” and all the things you did will be reversed immediately. 😉
Step 8: To save your new image, click the RECTIFY command and save using your preferences. If you don’t rectify, all the changes you made will be put to waste. So you know, RECTIFYYYYYY (insert ominous voice here)…
Step 9: Test your new image by opening a blank map and checking how it will look like. If you’ve done it right, it should overlay even without using the old map file. 🙂
I used ArcGIS version 10 for this construction. I might try using QGIS later and tell you how it’s done. Until then… 🙂