How to get exact real world maps with SRTM

[Stridor]

Disclaimer: This is an advanced tutorial for those who desire the utmost perfection in their maps made with MM and are willing to put in some effort. It requires some computer knowledge and skill. It is not for the novice map maker. You can still get good looking maps without doing this tutorial. I intend to just show the major points and not go into major detail here. It does require a few steps to get it done, but is still much faster and more accurate than doing it by hand. Once you learn the routine you can actually crank heightmaps out pretty quickly.

With that out of the way ...

SRTM = Shuttle Radar Topographic Mission, which mapped most of the planet with slant radar in feb 2000.

Whilst the continental US is in 1 sec of arc (~ 30 m res), most of the rest of the world including the Ukraine is 3 sec of arc (~ 90 m res). This is still better than anything else out there and whilst standard MM res is better (~ 15 m, MM high res is even better still ~ 7 m), if you can get an accurate height map at 90 m res then at least your height maps will start with an accurate base and you can then sculpt any extra features required in MM itself.

What you will need (all free)

1. Google Earth (GE - free ed) - note GE uses SRTM as well but doesn't have a heightmap export!
2. Google Sketchup (SU - free ed)
3. AutoDEM (www.autodemo.com)

This tutorial will demonstrate how to get an exact heightmap of the area you wish to model, and then load that HM into the MM.

[Stridor]

First let us select an interesting part of the world the model.

Most of the Ukraine is flat so I thought I would go with Switzerland to show off mountains! (This technique works for any area however)

Here I have framed in GE roughly what I want to map (note the map scale legend, I will frame the map exactly shortly)

Note how I have turned all GE Layers Off

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Ok now leave GE alone and start SU.

Press the import GE image button (shown). This will bring the displayed GE image into SU.

Next draw a 1000m x 1000m (enter “1000m,1000m” into the measurement box with the rectangle tool in operation) square and then drag it precisely over the GE area image you want to model. This will give you a 1:1 representation.

Now press the export to GE button.

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Now you will see your map square area of 1000m x 1000m overlaid exactly on the GE map.

Remember to turn all the layers off (including terrain)

[Stridor]

Now type “seamless.usgs.goc/index.php” into your browser. Please Allow Popups for this site.

Thank god your hard earned US tax dollars are allowing us foreign types to get free SRTM data [;)]

Press the “View and Download International Data” area.

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Once the next page has loaded click on the “Define Download Area By Coordinates” button

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You will now be taken to the data coordinates entry screen. Minimize this screen for now.

Go back to GE and zoom into the top left corner of your map as far as you can (I got down to 3m AGL – the closer you get the more accurate your results will be). Next move the cursor over the very top left corner of your map and then record the lat and long cords. You will need to feed these into the map server data entry screen. Next do the same for the bottom right corner of your map.

You should now have top left long/lat and bottom right long/lat values.

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Now enter these data points into the coordinates screen. Be careful to get your hemispheres correct!
Press the add area button when done.

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You will next be presented with the following screen. This is because you haven’t yet told the map server what format you want your SRTM data in yet.

Click the “Modify Data Request Button”

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Ok you will now be at the USGS Data Extraction Options Page.

Scroll this down to the bottom and make sure that at least the SRTM 3 arc sec option is checked. You can check the others for higher res feeds if you are doing USA maps. Also ensure GeoTIFF/ZIP/HTML is select from the drop downs.

Press the “Save Changes & Return to Summary” button when done.

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Now press the download button to download your SRTM map data.

Open the download ZIP then extract the special TIFF file out of it. It will have a number name like "01017400.tiff"

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Ok

You can close the map server now. You can close SU now. You don’t need to save anything from SU.

Open AutoDEM (AD). File->Open->DEM then select your SRTM tiff file you downloaded.

AD will display a small color heightmap. You can zoom into this to make it bigger. Note the layer control box. Press the “elevation range button” and you will see an elevation range bar to the right. The listed heights are the maximum and minimum for your map. Remember these as you will need to feed them into the MM during the heightmap import.

Note that the map is not perfectly square. This is because the pixel res is different between the X and Y axis. ??? This was because the shuttle cut out “swathes” of ground area with the imaging radar for each orbital pass ???. In any case can fix that little oversight later in MM.

[Stridor]

Next select Filters->DEM->Color from elevation. This will bring up a confirm dialog. Ensure you check the Color/Grey Level box here. Press ok.

Check the work layer. Unselect the DEM layer and you will see your colored HM now is in the correct greyscale.

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Now select File->Save View (save as a png) and type a name for your HM.

As of MM 0.9g, the MM needs HMs to be in bmp format. I will add the ability for the MM to support png HMs in future, but for now we need to convert the png HM from AutoDEM into a bmp. You can use any program for this (even MS paint) simply load it up and then save it as a bmp (eg My_HM.bmp).

There seems to be a bug in AutoDEM saving to bmps which is why we need to do this little (unnecessary) conversion.

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Now startup MM.

I will be using Rick’s WinterTemplateRR1 as a basis so I load this map up fist. Then I click the load H/Map: button and browse for my hieghtmap.bmp I saved.

Note: MM will complain because it is not in the correct 65x65 format. Just ignore it for now. Notice that after it loads it looks a bit blocky. Remember this is because the underlying SRTM source res is only 3 sec arc. For this reason you will probably want to do 2-3 smoothing passes on the map to make it look more natural.

Remember to type in the minimum and maximum heights from the elevation scale you saw in AutoDEM. Then build your map!

[Stridor]

This is how the map looks in SE with Rick's winter coat and 3 smoothing passes.

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And here is the final shot showing the map overlaid on GE. This confirms how exact this method is (and also the fact that GE is using 3 sec SRTM data as well). Note how the roads follow the curve of the mountains almost exactly.

Well that is the end of the tutorial.

Hopefully this is enough to get you started on making your own anatomically exact heightmaps.

Perhaps I can write some interface code in the future for MM to talk to the USGS map server directly and save a few of these steps. But that is in the FAR FAR future.

Regards and merry christmas all!

Stridor.

[junk2drive]

Thanks John for putting together such a detailed tut. I've played with some DEM stuff and it is not easy to figure out how to use different programs together.

[Mraah]

[X(] WOW !!!

I feel outta my chair after reading this !

Thank you very much Stridor for explainging how to do this.

Definately needs to be a STICKY ... and get rid of my old scruffy sticky.

Rob

[Stridor]

As an update to this I am working on an even simpler method of importing DEMs into the Map Maker.

I don't know if I will be able to achieve it but the plan will be to supply a local dem map (specifically a SRTM tile) of the kharkov region by default (actually a *very large part* of the ukraine. I will also show you how you can get other tiles if you want to do maps elsewhere in the world, each tile ~ 40MB - there is a very easy way to do this, much easier than I have shown above). Then when you are in MM you will be able to enter lat/long coords of what you want to model (I have found a way to do this easily in GE without the need for SU) and MM will scan all the master DEM maps (tiles) and cut and massage the data into the correct heightmap form. It will even calculate the correct max and min heights for you.

How cool would that be!

Anyway it is a lot of work so it will take some time to implement.

Regards

S.