Using SRTM

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Using SRTM

Postby altug » Tue 14 Mar 2006 - 00:42

Hi Barry :D

I was posting this as a reply to your last post which appeared under General in 13th of March but it sudenly disapeared so I put this as a new topic :D

I did Gocek scenery based on your tutorial and SRTM data. Hill heights distances and Latitude/Longitude appeared correct. However as you mentioned for the lagoons, small flat islands are not successful. Also I have been to this area and still there are unpleasant details at the result scenery. I am looking for a way to mix manual drawing with SRTM but for the moment no idea how to do it.

I can share an experience :idea: to get correct scale + correct Latitude/Longitude :

This is related to the following part of your tutorial:
" At the same time there is a Merge and Resample Box.
I click "any size" and make it 2000 for X and Y as, using a square dem map, to get a finer resolution and smooth mountains. I assume this is height points and the maximum is 2040 or therabouts, (I think) in the Scenery Designer.
Click OK and a message appears to say successfully installed or similar
. "

For Gocek at 3Dem I "select smaller area" from SRTM (N36E28).
At VT Builder at the "merge and export elevation" step comes up the "merge and resample" window.
At this point it is better to observe first the values of "size at that density" In my case it was 213 x 256.
Than after picking "Any Size" I put 2048 (max pixel for DEM builder area) for Y. For X i did not put again 2048 which would make it a square but instead I but 1704 which keeps the ratio of 213 x 256 for 2048.

When the scenery finished following all your tutorial steps, I compared the longitudes/latitudes plus distances between several points with the printed maritime map of the area. Amazingly correct :)

So first observe "size at that density". Than at "Any Size" put 2048 for the bigger value in "size at that density". It can be sometime X sometime Y depending on the form of the "selected smaller area" at first step in 3Dem. Finally for the other axis put a value which keeps the same ratio for 2048 .

With this you get perfect scale with correct longitude latitude.


As I told you my concern is loosing details...such details that I would not miss by drawing (I guess :roll: ). Left screenshot is from the first scenery I did manually without the correct heights. Right screenshot is from the SRTM scenery with lots of lost details, disapearing islands:

Image Image

The first pic is a download from internet. Looking from North to South, down there is Gocek town with its islands and bays souther. 2nd screenshot is from the SRTM scenery. I tried to put the camera at the same place as in the first pic :D . SRTM gives a general feeling of the area which is not bad but at details it is not good. For someone who has been there or who can compare the real pic it is full of lost details even some wrong hill heights etc...

Image Image

That is why the best would be mixing manual drawing with SRTM data. The problem is SRTM's resolution is not enough yet. Do you know any other SRTM source which makes closer earth pictures with higher resolution?

And I have another BIG QUESTION :roll: :roll: :
At Dem Builder, when opening the .bt file (made in VT Builder) in the "DEM Header Information" window there is this REFERENCE DATUM (huge list)....which reference datum to choose depending on what? Any suggestions? My printed map is ED 50 but in this menu there are several ED 50 starting from ED 50, EUR A to ED 50, EUR T. What to choose? Any comment would help...

Thank you for all the efforts and sharing you commit. :D

altug :D
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Postby Barry » Tue 14 Mar 2006 - 02:36

Thanks Altug.
Sorry about deleting the posts but had reached a stage when I was wondering why bother with no help or input.

Had written a few posts but thought it best to delete them as I was getting nowhere with anything, so you have saved the day.
Getting a bit late now but will post again tomorrow.

Regarding your questions I think the heights serm to be about right as you say.

What you have got to watch is to draw a depth right next to the land even if it is sloping down. If you don't the land at the edges tend to flatten out.
Each contour slopes down to the next. So if land is say 4 metres at edge and you put -3 a little way away, you end up with an extra bit of land added.
To get the sea back set no data to zero but you probably know that.

Just made Conway river and N.Wales again tonight and had the same problem.
Puffin island a huge lump of rock ended up with sloping sides.

I did try the merge as you said,, but was't quite sure what I was doing and why, or what order x and y was in.
That is why I tried keeping map as a square.
Will re-read your post about it again, but what you say seems the best way

I kept the VT just the same.
Jeroen mentioned don't alter any settings.
You can fine tune it with Sceney Dem designer as one scenery was about 20 perccent too small and alter spacing

Bit lost to be honest as you end up with pillars and blocks if you leave VT at the default 257 and alter heights in Scenery Dem.

Also a land map appears different to what we are getting, if you change the to episiloid or whatever in 3DEM map then, looks correct shape but gets squashed in.

I think you will have to write me a turorial as just guessing most of it.
Have written to Jeroen as Stentec and the posts have been passed on to him so maybe he will have a few answers?

We are Ok with mountains and islands like the Balearics but rivers and flat areas are "difficult".

I don't know how to put a river onto the greyscale map as you can't see to place it clearly, although you can set depths just the same on it.
Mountains of Wales are lovely now as kept the scenery about 20 miles with 2000 spacing in VT.

What shape is your map as Scenery Dem Designer seems to give different spacings if not square, and don't seem to make sense?
Maybe it can only make square sceneries?
Will study your post in more detail.

Thanks again.

P.S Perhaps You have water level to high and lower it in scenery dem to say minus 3 to get edges of land back. The satellite data could be reading at low or high tide. May not make much diffeence, but it does for my area at 9.3 tide.
Is the water set to 0 in 3DEM . If higher it drowns the land edges.
If altering it remember to make no data the same value to get the sea back.

PPS. You can alter the greysscale map but difficult finding the area.
Use a background map made from 3DEM in the Scenery Dem Designer.
Rub out using 0 as no data to get pink areas back and lay contours.
3DEM will save bmp etc. I tied that method for my rivers, but still "tricky".


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Postby Barry » Tue 14 Mar 2006 - 04:21

Altug Hi

I made a quick version of the scenery.
Only took half an hour as you had the co-ordinates for map.
Anyway went round all te islands with -20 depths and covered pink sea in -20 depth contours.
I think you would need to set some depths around the islands whimh makes them bigger?

Looks quite nice.
Nice easy way to visit any part of the world.
Left it at NAD 1927 NAS - C. Didn't alter any settings.


Last edited by Barry on Fri 12 Jan 2007 - 22:59, edited 1 time in total.
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Postby Barry » Tue 14 Mar 2006 - 14:01

Altug Hi

Just to add a few more bits.
When I patched my version of your srtm scenery, I think it removed 41 voids.
That might have been the full srtm.
What puzzles me, my sceneries are not quite the same shape as OS map.
I changed it in 3DEM from UTM epsiloid to UTM projection as terrain looks about the correct shape instaed of squashed.

Once you set the sea, which is at zero in 3dem, to no data in Scenery dem designer, you can adjust and mould the land outline on the blank pink part, by laying depths as threads, etc..
I think that will make all the difference.

To delete some land set the "pencil/pen" to 0 then rub over the part you want to reomove with "pencil "pixels. Then set "no data" to 0 to make that part blank pink dem in scenery dem designer.

You can get the heights by hovering the mouse over the 3 dem map.

Make a bmp of the 3dem map. colour any parts you want in to show the depths you want or beach,
set as background against the greyscale map when sea set to no data showing the pink area, etc and lay your new depths. As long as you have the island terrain height about right you can shape it any way you want.

Set the depth threads very close to terrain for deep water and a good minus depth and perhaps move a bit further away at a say minus 3 for beaches. That however will add a bit more land . I suppose its just a case of experimenting to get the shape you want.

For my version of your scenery I set the x/y as you said, as a percentage of the version I cropped. 2048 mximum, as the longest side.

Also not sure when you convert to scene and crop the pink edges, does that reset the new cropped size automatically?
I think I will keep clear of setting the terrain size heights and spacing in scenery dem designer and rely on VT terrain settings. May remake Isle of Man using the percentage method.

You probably know most of what I detailed but just going over the way I make the sceneries.

Maybe Stentec will make life easier for us come November witH new Sailsim 5 and new scenery Scenery Designer.:wink:

Regarding other srtm files I think there is 30 metre, but not sure of that.The main one is version 2 though 90 metre?, as has been corrected for cloud cover and voids, etc?


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Postby altug » Fri 24 Mar 2006 - 01:50

Hi Barry :D

Thank you again for every info you share.

As I mentioned SRTM gives a general feeling of the area which is not bad and can impress those who have not been there. But at details it is not good.

See how is these small islands are in reality:



Also as we have a fix 2048 pixel limit at Scenery Designer, bigger the region we want to cover for scenery making, bigger the details we are loosing unfortunately. For example 1 pixel representing 100 m. vs. 1pixel representing 1000 m.

Perfect solution would be:
1) Closer to earth made SRTM data with higher resolution ( wishing from NASA :D ).
2) Bigger space in pixels in Scenery designer. ( wishing from Stentec :D )

With what we have in hand I think the methods for manipulating SRTM sounds perfect in your tutorial, I mean this method of redrawing pink areas. With your method of seting no data at zero I will redraw the coast line and try to get back the small islands :D, fix the beaches and depths etc.

About the shape of the sceneries it can be any rectangular shape not necesarly square. If it is not a square the only abnormailty appears during the game at scenery pickup dropdown menu, the map of the scenery looks streched, because the pic window for it is a fix square.

Also now able to put SRTM plates together I decided to have the scenery not as Gocek but the whole Gulf of Fethiye :D. I will write about it next :D

Thank you again :D
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Postby Barry » Fri 24 Mar 2006 - 02:47

Hello Altug.

I tried a long strip, in the iom to Scotland scenery I just made. and it worked out OK.
However I changed from UTM ellipsoid in 3DEM, to UTM projection otherwise the scenery has a squashed-up look when compared to actual map of area.

USA area is I think 30 metre and Europe an world 90 metre.
So probably won't pick out very small islands, etc, although most of the heights look about correct.

In no data at 0, test with water at 0, and you will see any lines that shouldn't be there shown in black. ie. perhaps if you make a mistake when placing them, the rest are blue. Saves a bit of work before you generate the scenery, as easy to go over them with "pencil" at no data 0, and correct or adjust them.

Best to experiment the best way to use no data but be careful you don't obliterate all the terrain at one height. I keep making a back up "just in case"

Trees would make the islands look better and add some "depth" but again we can only use 32 bit texture in m/b simulator.

May be best to make a series of cropped sceneries, rather than one big section otherwise pixels too large, although quite good up say under 100 kilometre. Its an easy matter to start one, where the last finished.
Also easier as you can then keep going back to adjust the dem map if you wish.
We could have a nice library of sceneries eventually, following around the coastlines, adding another when time permits, etc.

If you make deep depths very close against island edges they would have steeper edges, but depends on what depths you are using. and how accurate you want the depths.

Again you probably know what I have mentioned, but that's about the total I know at present.

P.S. Just been looking at my Isle of Man Scenery.
Apparently it is probably impossible to make cliffs, or steeply sloping cliffs, that go down to a sandy beach. If you make the water minus a metre or 2 at the base of a cliff height, the ground slopes from top of cliff height down to the shallow depth, just adding sloping land. with no trace of a cliff. Suppose you could make them as 3D models.
Been having a look at google images for each port/harbour and mine and reality look very different.

Maybe when we get an updated Scenery Designer we can get more realism.

Thanks Barry

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Postby Barry » Sat 25 Mar 2006 - 14:33

Altug Hi

Been having a think about what you said about loss of detail and decided keep my future sceneries to a maximum of 50 kilometres with maximum points at 2048 as you suggested.
That gives enough density of pixels in any one area to work with.

To solve the missing rivers I will set the no data say to 7 plus, if say the tide is 7 metres in that area, This will pick out the outline of the edges of the river up to tidal height inland. Then make the middle part no data 0 with "pencil" symbol to get the blank dem area to work on for the rivers.
Then replace all the terrain pixels at 7 plus that get removed at the same time. Probably takes an hour or 2 but at least you get the rivers back.

The only way with the missing tiny islands, rocky outcrops, complete missing small harbours and inlets is maybe refer to an OS map at 1:25000 scale and put all the features in over the grey scale map with no data at 0.

Although you could use say 1:50000 scale not quite as detailed.
I think the OS map will be more useful to me than the sea charts. Although may use those for general reference if I have one for the area.

Maybe Stentec will come up with some "ideas" in the future hopefully.

Also added these details to my website srtm tutorial.

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Postby Barry » Sun 26 Mar 2006 - 02:07

Altug Hi. re srtm
Remade IOM for about the 3rd time and memo to summarise.
Mountain are quite good and reasonably smooth.
Using 0 no data AND 1 metre "no data" - starting depth about -3 and -9 kept VERY close to terrain, otherwise it may add land.
1 metre "no data" enabled the water to divide a small island as it should be. max 2048 pixels as you suggested.

Keeping sceneries now to under 50k approx as spacing then only 25 metres.
Added a few small islets by ref to maps and lengthened some small rivers. I don't think we can improve the procedure any more at present with what we have available, so will stick to this system with 50 kilometre sceneries following on from each other. No more trouble than making 1 very large scenery with reduced details.


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