SRTM versus USGS DEM Data

[Signalmaster]

With a relatively easy way to convert standard USGS seamless DEM data in the RS required HGT format, why would anyone use any tolerance SRTM data?

These two images should clear up any doubt which is more accurate. The view is lower Manhattan Island in New York City. The data with the grainy surface is SRTM data, the smooth surface is USGS DEM data.

The graininess is caused by the fact that radar can't "see" through objects on the ground. That includes the skyscrapers located in NYC. This occurs all throughout the SRTM dataset. Also, there's voids in the data, as represented in the white splotches. If your using SRTM data for hilly terrain and oneof these voids occurs, how do you fix it?

Good Luck
Hank

[landnrailroader]

My own preference is to massage the SEAMELESS USGS data with MicroDem and go from there which is what I assume you did. Now it happens that in the Clifton Forge, VA area there was no discernible difference in the two, but in big cities, I can see where there is a definite advantage to using the USGS DEM data.

Jerry Sullivan
(landnrailroader)

[livercup]

I downloaded the srtm1 files from the site suggested by RS. Is the seamless data really that much better? I have 1/9 and 1/3 NEDs available for my area. If so, how do you convert it? I searched the forum and couldn't find it. Found a tutorial on converting Canadian dem. Same thing? I know I have seen this discussed but I would appreciate it if you could point me in the right direction.

Tom

[Signalmaster]

The Canadian tutorial is just fine.

For US DEM's, go to the USGS Seamless Data site and download the data for the area and at the resolution you want.

Find the MicoDEM freeware site and download MicoDEM to 'see' your data. Export the displayed data as a BIL file.

Refer back to the tutorial for the site to download a BILSRTM convert utility.

Your file will end up as an HGT file which you store in the DEM/SRTM subdirectories in the root of RailSimulator.

Enter the World Editor and start work on your new route. Pressing 'T' will will bring the terrain data into your route every few squares. Each activation of T terriforms a certain size square or tile which right now I don't know the size. But it has to be done frequently and saved.

There are some caveats here. One is if you reapply the T action over an area you may have added some texture effects, they might disappear.

This is a basic outline, search the subjects for other tips given in the forum.

Regards,
Hank

[livercup]

Thank you for your response. Will using the neds give me better terrain that the srtm1 data, if I do it this way?
Tom

[livercup]

Okay, maybe a dumb question. After actually reading the tutorial I see that the maximum detail I can get is 1 arc sec. I would love to see support built in for using the 1/3 and 1/9 neds as, I suppose they would create greater detail.
Tom

[zbobg]

Yesterday evening I downloaded the seamless USGS data for a route that I had already done the terrain for using the SRTM1 data.

After converting the USGS data to .hgt files I applied that terrain over the already done SRTM1 terrain. There was very little to no difference in appearance. The USGS data was about 1 meter lower but all the landscape details were equally present in both datasets.

The fact is that much of the USGS data is obtained from or merged with the SRTM data and then presented to the user. So there will be little difference except in unusual cases.

The advantage to using the program MicroDem with the USGS data is that you can preview the dataset and edit it if necessary. You might search for voids in the data and fill them with an appropriate median height value to eliminate the occasional 'big deep hole' in your scenery for example.

[beluga]

[QUOTE=Signalmaster;1372270]The Canadian tutorial is just fine.

For US DEM's, go to the USGS Seamless Data site and download the data for the area and at the resolution you want.

Find the MicoDEM freeware site and download MicoDEM to 'see' your data. Export the displayed data as a BIL file.

Hank,
Several questions please
1. Which type of data is to be Dl from USGS Seamless Data site ?

1" NED
1/3" NED
SRTM Finished 3 arc sec
SRTM Finished 1 arc sec

2. How do you use MicoDEM to view the files?
The files I dl are .adf files-is this correct?

TIA
Jim

[zbobg]

RailSim only uses 1 arc second (or greater - like SRTM3) terrain files.

So you need only download 1 arc second files and if you click the "Modify your Order" button on the USGS download site you can change what it is you download.

So, in the modify your order window, scroll down until you see your current 'order'. Select (checkmark) the 1 arc second NED checkbox, on the right change the file type to BIL which is the same format that RailSim can eventually use. The Geo... files are higher resolution but RailSim cannot use them, you'll simply have to convert them finally to BIL format. So download the BIL format to start with. The ArcGrid files (.adf ?) are not usable by RailSim nor by Microdem, that is the default format so you MUST change that particular entry.

You still need a BIL to SRTM converter. There is a link to one that works perfectly inside the SRTM_tut.zip available in the Train-Sim download library.
It is this file BILxSRTM.zip (http://www.cplus.org/rmw/bilxsrtm.zip) which is a direct link to the download.

If you have downloaded the BIL format files you do not need to use MicroDem at all, just convert the files.

I found it was easier to define my selection of area to download from the USGS site by using the coordinates method. Just define the rectangular area by the four corners expressed as decimal longitude and latitude.

[beluga]

I'll try it.
Thanks for taking the time to help me.
Jim

[wildh0rse]

I went to the usgs seamless site - the map wont display for some reason thugh everything else does. I treid resetting my screen resolution - any thoughts? Using IE 6.0

[livercup]

Sometimes it takes a while, some times the site is down for maintenance. I've had the same problem before and it always ends up working...eventually.
Tom

[boleyd]

Just to concur - I spent some time Sunday trying to get some downloads of scenery for my flight simulator but it was a "rocky" experience.

[wildh0rse]

Thanks, my friends - thought it was me! ;)

How may I get the best resolution from the files, ie what settings for maximum terrain detail?
Is there any sort of difference for rural/mountainous terrain?

Oh - and here's one for all you MSTS alums :D - is there any type of 'distant mountains' type of option?

[rjdiii]

The advantage to using the program MicroDem with the USGS data is that you can preview the dataset and edit it if necessary. You might search for voids in the data and fill them with an appropriate median height value to eliminate the occasional 'big deep hole' in your scenery for example.

Occasional? Try doing the terrain for Glenwood or Byers canyon in Colorado. It's just one "big deep hole" with occasional terrain. The Sierra Nevada has good places and bad places, Cape Horn and kilometer lengths of the American river canyons are a "big deep hole".

SRTM data just doesn't cut it for mountain railroading. There is a fundamental problem with side looking radar looking down into deep narrow canyons similiar to problems in Manhattan. RDSL has asked what other formats would people want and may be supporting others in the future. So long as it's something MicroDEM supports I'll be happy.

The sim itself can handle much more detailed terrain than the SRTMs provide as witness Deanville's 1/3 arc second terrain. He's waiting to see the MKII update before deciding whether to build and release a separate terraforming utility.

Richard

[deanville]

AFAIK the horizontal terrain resolution in KRS is 8m. The horizontal resolution of 1 arcsec data is about 30m, 1/3 arcsec data about 10m, and 1/9 arcsec data around 3m.

I see slight improvement in KRS terrain using 1/9th data rather than 1/3.