How To Generate Contours Using Global Mapper
How To Create Contours Using Global Mapper
1. First you must select a target. Since I map islands, my target island is Sadoga-shima JA. With the selection of the target, you need to gather the lat/lon coordinates for the NSEW extremities of the island. Here I gather the coordinates using Google Earth.
2. Now you head into the SRTM3 version 2 area to snag your SRTM files. I use an index map that I have on my desktop to determine the region from which I will download my files. The region I need is Eurasia.
3 & 4. I need two files to cover my target area. the first is N37E138.hgt.zip and N38E138.hgt.zip. These are relatively small files (778 KB and 157 KB) but the display is stunning. I download the files to my desktop and then place them into my EVS Islands work area.
5. After the files are securely stored, I open Global Mapper (GM) which will be my contour generating software. First I select File menu and Open Data File(s)...
6. GM allows for importing of multiple files. I select both files. I do not have to unzip them as GM handles that task as well. I merely select them, select Open and the files load.
7 & 8. The load is fast. I happen to be using a display called HSV Shader. I can select certain colors and have them shade to give me my choice elevation display. Once my SRTM .hgt zip files are loaded, I zoom in to include only my target area on screen. It is not necessary, but it is my work habit.
9. Now for a slightly funky step. It is my experience that SRTM elevation data reads elevations in the open ocean where there is no land. I suspect it is reading the sea level, which is higher then SRTM's 0 meters level. (I think this makes sense.) Anyhow, if I change the water level to 10 meters within the Configuration Menu/Vertical Options, most of the problem contours are handled.
10. Now comes the fun part, making contours. Select File and Generate Contours.
11. You now define certain characteristics of your contours. This Contour Options is the default. I usually stay with the default, unless something really wacky comes up. 50 meter intervals, although busy at 1:337,200, looks very good at 1:50,000. I typically Generate Spot Elevations at Min/Max Elevations. The two choices below it are default options.
12. I didn't display the Simplification Option. It is self-explanatory - you either get smoother or less smooth contour lines (more or less vertices). The Contour Bounds gives one lots of choices on how to select your target area. Since I only include my target area on my screen, I select the second option. Sometimes I Draw a Box... around my target area. Select OK.
13 & 14. Select OK... I had to capture this next screen fast. The contour creation process typically lasts about two to three seconds for my islands. Very large areas will take a greater amount of time.
15, 16 & 17. And here are some of the contours. This display, because of the scale, shows only a few major contours. However, once you zoom in, all of your newly created contours are there to behold. There is cleanup that needs to happen. Sometimes contours extend across open water due to SRTM misread. Also a few scattered 0 meter contours should be selected and deleted.
And that is how you are able to use Global Mapper to create contours. It is easy, fast and mostly free, except for the cost of Global Mapper (about $200).
Enjoy!
Labels: contours, Global Mapper, hgt file format, map, SRTM
