Scale and Projection Questions
choppinlt
Traveler
OK, so I've been told that the scale bar for SRTM data in FT3 exported to CC3 is not reliable and to ignore it. My goal: to overlay the SRTM contour data over a corresponding map. After playing around with the maps it has been an exhaustive process, so the lack of scale for the contour data is a significant issue for me. Question 1- is there a way I can draw something/create something in FT3 to use as a scale that can be imported to CC3? i.e. can I create my own scale?
Q2-I'm trying to match the SRTM map with an old topo. I believe the 1:100,000 topo uses universal transverse mercator projection. Before I export the SRTM data should I convert it to a Mercator projection before exporting it? Or would that make any difference?
Thanks!
Best Answer
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WeathermanSweden
🖼️ 3 images Surveyor
The size of a SRTM field is always 1 degree x 1 degree.
If you export a complete singel SRTM field from FT3 to CC3+ the distance between the northern and the southern limit of the resulting map is always approximately 111 km (= 1° / 360° * 2 * PI * 6357 km) - where 6357 km is the radius of the earth at the poles.
You get almost the same results when using a mean radius of the earth (6371 km) or the equator radius (6378 km). With this in mind you can estimate the distance between the western to eastern limit of the map to 1° / 360° * 2 * PI * 6371 km * cos (latitude in degrees). This is not totally mathematically correct - because the radius of the earth is a function of latitude, too (but this can be neglected here).
At the equator the distance is 111 km and at the poles it becomes 0 km.
Export of a part of a SRTM
When you only export a part of a SRTM you have to note the "upper left" coordinate and the "lower right" one in the FT3 map and then you take the differences between the latitudes and insert the result in the first formula instead of "1°". Take the difference between the longitudes and put the result in the second formula (replacing "1°"). Take the absolute values of your differences of course.
Regarding your second question: When changing the projection the shape of your map changes. But as long as latitudes and longitudes in your map still are approximately perpendicular to each other - you can use the method I described before to estimate distances in your map and from that the scale, too.
Greetings,
André
Answers
There are two ways that I know of, these are how I do it.
1) You can export the ft3 map as a cc3 file. Load it into cc3+, and use Info -> distance to see the size. Add an appropriate scale bar.
2) I took a continent in FT3, exported the png, loaded an island/ocean template in CC3,CC3+ wasn't out yet but this should work as well for CC3+, imported into the template of whatever size I decided it would be. Add an appropriate scale bar.
Other folks likely have better ways.
@choppinlt
The size of a SRTM field is always 1 degree x 1 degree.
If you export a complete singel SRTM field from FT3 to CC3+ the distance between the northern and the southern limit of the resulting map is always approximately 111 km (= 1° / 360° * 2 * PI * 6357 km) - where 6357 km is the radius of the earth at the poles.
You get almost the same results when using a mean radius of the earth (6371 km) or the equator radius (6378 km). With this in mind you can estimate the distance between the western to eastern limit of the map to 1° / 360° * 2 * PI * 6371 km * cos (latitude in degrees). This is not totally mathematically correct - because the radius of the earth is a function of latitude, too (but this can be neglected here).
At the equator the distance is 111 km and at the poles it becomes 0 km.
Export of a part of a SRTM
When you only export a part of a SRTM you have to note the "upper left" coordinate and the "lower right" one in the FT3 map and then you take the differences between the latitudes and insert the result in the first formula instead of "1°". Take the difference between the longitudes and put the result in the second formula (replacing "1°"). Take the absolute values of your differences of course.
Regarding your second question: When changing the projection the shape of your map changes. But as long as latitudes and longitudes in your map still are approximately perpendicular to each other - you can use the method I described before to estimate distances in your map and from that the scale, too.
Greetings,
André
Thank you @Weatherman, that is very useful! I can put this to good use in the future. In the meantime I found a different, perhaps more empirical method. In FT3 I measured across the bottom of the entire land mass. This differed because I had some coastal areas. Knowing the distance in FT3, I was able to export to CC3 and scale accordingly. I'm not sure if this is the best or most correct way, but it appears to work!
Regarding the projection, I played around with it some and my early judgement was that it did make a difference. Making the SRTM mercator appeared to make a better fit for my purposes. I havn't exhaustively tested it, but that is my early conclusion.
I have another related question that I will post in another thread. Basically I am trying to figure out how to marry 2 SRTM maps together.