Home > 2. Marine GIS > 2.14 Image Georeferencing
2.14 Image Georeferencing in Saga
New Version of Saga: Usually a new version of Saga is quickly assimilated into these exercises. The latest release, Version 2.1.2, is so radically different that it will take some time to catch up with revised exercises. Please email me immediately whenever you find an old exercise that simply cannot be accomplished in the new Saga, due to big changes in the properties, etc. I will try to provide fixes within about 2 days.
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1. Open the example image, a satellite view of a typhoon in the northwestern Pacific. You can see that, fortunately, the image has a labeled graticule we can use in the georeferencing process.
You can save a copy of this file in the folder DATA > OCEAN > JTWC NOTE: At this point, if you wish, you can cut away the black borders with any graphics editor. |
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2. Run Saga. Then select MODULES > IMPORT/EXPORT IMAGES > IMPORT IMAGE
Then click OK to load the file. |
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| 3. In Saga, this new grid or raster object appears. |  |
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4. You can use ADD TO MAP to see the image in Saga. Notice that the cursor locations (shown along the bottom) are not in geographic coordinates...they appear to be pixel locations on the image.
We must "georeference" the image to make locations on the image have real-world longitudes and latitudes. |
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5. Select MODULES > PROJECTION-GEOREFERENCING > CREATE REFERENCE POINTS.
Leave these settings, and click OK. |
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6. Use the ZOOM and PAN tools (along the top) to locate known locations on the image. In our case this is easy because we have a graticule with labels.
When you find a good location, use the ACTION tool (the black arrow) to select the exact point you want and click. |
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7. This window opens, where you should enter the known coordinates of the point.
Then click OK to capture the point. NOTE: The results will be a disaster if you use the wrong signs for the latitude or longitude, so be very careful. |
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| 8. You must repeat this process at least four times, for points scattered around the image. If the image is obviously projected, then more points than usual would be a good idea. If you don't have a graticule, then you need a table of lat/lon values for known locations, such at tiny islands, highway junctions, city centers, etc. The more good points you have, the better will be the calculations. | |
| 9. When you are finished finding reference points, then open the MODULES menu along the top of the map, and turn OFF the module by clicking on CREATE REFERENCE POINTS. |  |
| 10. Answer YES. |  |
| 11. Now you can see a new points shape has appeared. You can plot it if you want to see your points. |  |
| 12. We need to combine these locations with the image. To do that, select PROJECTION-GEOREFERENCING > RECTIFY GRID |  |
13. Make these settings:
Then click OK.
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14. This window appears, containing the results of all the calculations and your input, above.
We simply accept these values, but it's obvious that if you know what you want you could make some changes here. Click OK when you're ready to continue. |
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| 15. Now you can see a new raster/grid with a system description that looks like good geographic locations. |  |
| 16. You can use ADD TO MAP to see the newly georeferenced grid. |  |
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17. The colors are not like the original, so on the right (in the properties) select COLORS > TYPE > RGB (stands for red/green/blue)
Then select SETTINGS > APPLY |
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| 18. Here's the final, georeferenced satellite image, with the published colors from the original JTWC image. |  |
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19. Whenever you georeference any data layer, its a good idea to add a known reference map, preferably global, to confirm accuracy.
Here the World Borders Map (see link above) has been added to confirm the properlocation of the land bodies and faint green coastline in the JTWC image. |
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20. Now we need to save the new product. In Saga, select MAPS > THUMBNAILS (bottom left corner) and find the final map.
Right-click on it, and select SAVE AS IMAGE. |
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| 21. Navigate to PRODUCTS > SAGA > GEOIMAGES and enter the filename typhoon_nari_24w_20131015_jtwc_saga.png | |
22. This window opens with many settings.
Then click OK. |
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23. All of these new objects appear for use in various ways:
In other MDL exercises you'll be referred back here for georeferencing work, so expect to be asked for one or more of these product files. |
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| 24. And just to prove how useful they are, here's a Google Earth map of the image. You can open it by simply opening the KML file, above, in your folder. |  |