Recreating real terrain from Google Earth

[Emx77]

I'm trying to recreate real terrain map but I have faced some difficulties. My goal is to use Google Earth and Google Terrain overlay in combination with AATF Power Toolkit (AATF PT) to recreate real terrain. I decided to go with 6 x 5 km map. I followed "Creating Maps from SRTM Data - Tutorial" and sucessfuly obtained XDM map with size of 1 degree by 1 degree square where bottom left corner coordinates are 43 deg. 47' 10" LAT and 18 deg. 58' 26" LONG. Next step is creation of elevation map in AATF PT and that is where real problems start.

1) When "Map Production Information" dialog is shown if I accept all default values and simply clik "OK" button, I get 50 x 50 km map. So far so good. But, as I already stated, I want smaller 6 x 5 km map. After adjusting map dimension in "Map Product Information" dialog, the resulting smaller map is distorted along upper and right edge.



I am not sure why this disstortion appears? Also some of contours are disconnected. Is it a bug or maybe I need to change some of the default parameters in Map production Information" dialog?

2) Resulting map of 6 x 5 km have 2400 x 2000 pixels. Doing simple math I concluded that 1 km = 400 pixels. After adjusting Google terrain map (which have 20 m contour lines) to have same 1 km = 400 pixels ratio, I would expect to get exact match after I overlay Google terrain map over AATF contour map in drawing program, but it is not a case. They are comparable but not (exactly) matching. I assume solution could be in adjusting map construction parameters in AATF PT "Map Production Information" dialog, but I don't know how to obtain these parameters from Google Earth.

3) After I convert my map bottom left corner coordineates of 43 47' 10" LAT and 18 58' 26" LONG in seconds I get 157630 LAT and 68306 LONG which does not correspond with 154800 LONG and 64800 LAT which are default values in "Map production information" dialog after opening XDM file. I wonder why these values are not matching and which values should I enter?

4) Having all of the above in mind, does anybody know how to recreate real terrain using Google Earth and AATF PT?

Many thanks in advance.

[Patrick Proctor]

The default values in the map dialog are the same, no matter what DEM you load, ith the exception of the lat long values. The lat-long values should be the bottom left of YOUR DEM. You will have to change them to the bottom left of the map you are trying to create (which should always be a higher number than the default values). If the default values for lat long in the map dialog do not match what you think should be the bottom left lat long of the DEM you created using SRTM data, you might need to recreate the DEM.

To get a map that EXACTLY replicates the terrain on Google Earth, you will need the m/s and alpha angle for the map section.

The m/s entry is, literally, the number of meters in every arcsecond of longitude. The Alpha number is the difference between true north and grid north in mils (6400 mils per 360 degrees).

You can determine this data in a number of ways, but the best way is by loading the USGS DEM in MicroDem. This is a free download available here:

http://www.usna.edu/Users/oceano/pgu...e/microdem.htm

Use the info function to get the grid transform angle. That angle will be in degrees, which you can convert to mils. To figure out the m/s, you may have to divide the total width of the map in meters by the total width of the map in arcseconds (3600 if you have a 1 degree by 1 degree DEM).

There is nothing to be done about the edge effects on the bitmap. It is a quirk of the drawing program.

[Emx77]

Pat, I would like to thank you for realy useful information. During, last two days I have been trying to use all of above information in order to recreate small portion of terrain. Unfortunately, I haven't succeeded yet as I am probably doing something wrong.

Originally Posted by Patrick Proctor

Use the info function to get the grid transform angle. That angle will be in degrees, which you can convert to mils.

In MicroDem function "Map information" gives following:

True north at top of map
DEM native datum: WGS72
--------------------------------
Magnetic declination: 3.5°
UTM Grid declination: -1.2°

At the same time function "Horizontal Earth Curvature" provide:

Point: N43°46' 0.01" E18°58' 0.00"
Grid true angle: -1.56

I noticed that -1.2° (-21.3 mils) is some kind of average for whole map area, while -1.56 (-27,7 mils) is value for bottom left corner of map. Also, with function "UTM-grid true north declination" it is possible to get value for exact spot on map (for example -1.54). I hope that these are "grid transform angle" values you have mentioned before. If so, I presume that best measure is one obtained through "UTM-grid true north declination" for map area I want to recreate. Right?

Nevertheless, I tried all of mentioned values on my map but resulting AATF contour map doesn't match Google terrrain map and there is always some discrepancy. If I entered right coordinates, mils and m/s, could there be any other reason for this discrepancy? Different map projections maybe? Also I didn't change default 8-digit UTM values and Grid Zone Designators in Map Information Dialog (as I presume that is enough to put right long. and lat. coordinates).