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Holger

SRTM for South America

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Hi all,On the JPL NASA public distribution site ftp or http://edcsgs9.cr.usgs.gov/pub/data/srtm/ for the 3 arc sec SRTM data there is new directory South_America. I was curious and watched one tile S33W071.hgt with the mountain Aconcagua. On the maps 6959 meters, in the hgt file 6928 meters. In high mountains again there are many voids (blue colour in this illustration): http://www.volny.cz/stoje/scen/s33w71.jpgHas somebody the experiences with the difference of these radar data in comparison with the true topographic data (the impact of the huge buildings, big forests with tall trees etc)?Cheers

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Good find.Good to see more SRTM data coming out.Someone should write a utility so that we can produce our own mesh files from the SRTM data... instead of having to purchase it from 3rd parties.I mean how difficult can it by to write a utility to drive the scenery SDK to produce the mesh files from the SRTM data?BTW does 3 arc second mean 90m resolution?Its going to be great flying around the globe with SRTM data and FS2004 !!!In particular I really want Philippines and Australia. Maybe also Papua New GuineaCan't wait.Matthew.

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Just looking at your picture again.That 3 arc second detail looks really amazing!!!Just have a look at that circular "valley" at the intersection of -32.5 and -70.65. Will be great fun flying into that!!!And exploring all those other valleys.Bush flying Chile style :) (or is it Argentina)Matthew.

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Hi Matthew,yes the 3 arc seconds are about 90 meters resolution. The voids in SRTM data causes that this is still for the scenery creators, which can complete the data with other sources. The interpolation could be good solution for the small voids. For the first view on the data, if you don

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That BGL looks great (shame about the null data :( ) but i think SRTM will be reissued later with that null data fixed.Did you need to convert the SRTM data in any way? Or can the SDK use it without any modifications?Are you able to write a command line utility that does the following :-* reads all SRTM files in a directory* stiches SRTM data together* builds the INF files needed for the SDK* runs the SDK to build BGL Im a delphi programmer... so maybe I could write such a utility... if you help me with the requirements for the SDK, it seems fairly simple!What size is that BGL? Can you upload it here so I can download it?Once again... looks great!Matthew.

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In using MICRODEM to correct holes, we can process this BGL[Destination]LOD = 9DestDir = "."DestBaseFileName = "s33w071"UseSourceDimensions = 1[source]Type = ElevS16LSBSourceDir = "."SourceFile = "S33W071.bsq"CellType = S16LSB NullCellValue = -9999Lat = -32Lon = -71NumOfCellsPerLine = 1201NumOfLines = 1201CellXDimensionDeg = 0.000833333333CellYDimensionDeg = 0.000833333333ScaleInMeters = 1.0http://forums.avsim.com/user_files/10859.jpg

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Howdy:really nice to see South American data - it's such a diverse (sub)continent. Ten years ago, I was setting up crude digital elevation models for southern Chile for a research project on wind erosion; we've come a long ways since then ;-)If the holes in a SRTM tile are too large to be interpolated in a reasonable manner, I've had good results with merging the SRTM tile with a GTOPO30 tile of the same area (resampled in MircroDEM to the same resolution).Cheers, Holger

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Hi Vlada:thanks for the pointer.In regards to your question: I found a scientific paper about SRTM, entitled "The shuttle radar topography mission - a new class of digital elevation models acquired by spaceborne radar" by B.Rabus, M. Eineder, A. Roth, and R. Bamler, published in ISPRS Journal of Photogrammetry & Remote Sensing 57 (2003) 241? 262.The authors verified a horizontal accuracy of SRTM data better than 3m (6ft) and a vertical accuracy of within 16m (48ft) in 90% of the data (that's the typical way of reporting the magnitude of errors in a statistical sample). That level of accuracy is better than for most topographic maps. Also, radar data does penetrate vegetation and is therefore especially useful for mapping elevation in tropical areas with tall trees.The authors also state that the "holes" in the data are due to "layover and shadows", which are a result of the fact that the radar signals are sent out and received at an angle. The bottom of steep valleys and canyons are not reached by a signal that doesn't come from straight above. Also, smooth surfaces, such as lakes and fjords, may bounce off the radar signal like a billard ball, rather than reflecting it back to the satellite. Apparently, the way NASA/ESA are going to correct for most (but not all!) of the holes is by merging two different passes of the Shuttle over the same area. End of lecture ;-)Cheers, Holger

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HiI downloaded some .hgt data for Chile, some of the work perfect when processed with the resample code you provided, however, I have some more questions:1) I also use Microdem, but I'm unable to fully remove the holes. Can you give me a step by step tutorial on how to remove the holes the best?What is the best & suitable method to save the final improved (hole-removed) DEM data, so resample.exe can use it? What kind of DEM should I seave it in?2) There are some .hgt files that seem to be corrupt, however, they process normaly like the others, but when I look at them in tmfviewer.exe, they just show a random noise and no terrain. In FS2002 they produce a complete mess with all these brown stings that rise high up to the sky. However the .hgt files look OK in Microdem, even exporting them from microdem and resampling them again doesn't help.Can you or anybody else help me with these issues??Thanks a million in advance!

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Yes I agree... Can someone show me where I can find tutorial for building mesh from SRTM data.Matthew.

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Dear All:Yes the data is amazing, yesterday i created Lima Peru, Anta huascaran, Tarapoto, Cajamrca mesh terrain and the final result are incredible the elevations are real and match with my scenery.I'm usinf LOD8 to keep high frame rates and have good detail, i'm making the lat download to make the Juanjui and Moyobamba Mesh terrain.Is incredible the detail showed in Microdem with the data, i see the andes like shark teeth in some parts.In the next day i will post some pictures of Huascaran and depth canyon.Near to Arequipo is the Colca canyon see it with Microdem, is incredible.Thanks NASA and USA!!!!!Alfredo Mendiola LoyolaLima Peru

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Hi Matthew,Nice imagine to fly above SRTM mesh scenery in the FS2004 clouds! The hgt file in my example was without any modification. The quickly prepared s33w071.bgl with the holes has the size 795454 B. I think it isn

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Thanks for that.I actually got that BGL working :-jumpyLooking great.Matthew

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Im trying to build a BGL of 2 adjacent SRTM files. But RESAMPLE just seems to run forever. Can you tell me whats wrong?Thanks.Heres my batch file:-resample dual.inftmfcompress dualu.tmf dual.tmftmf2bgl dual.tmf dual.bglHeres my INF file:-[source] Type = MultiSource NumberOfSources = 2[source1] Type = ElevS16MSB SourceDir = "." SourceFile = "S33W071.hgt" Lat = -32 Lon = -71 NumOfCellsPerLine = 1201 NumOfLines = 1201 CellXDimensionDeg = 0.000833333333 CellYDimensionDeg = 0.000833333333[source2] Type = ElevS16MSB SourceDir = "." SourceFile = "S33W072.hgt" Lat = -32 Lon = -72 NumOfCellsPerLine = 1201 NumOfLines = 1201 CellXDimensionDeg = 0.000833333333 CellYDimensionDeg = 0.000833333333[Destination] LOD = 9 DestDir = "." DestBaseFileName = "dualu" UseSourceDimensions = 1

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Doesn't matter, found my error. Should look like this[Destination] LOD = 9 DestDir = "." DestBaseFileName = "dualu" NorthLat = -32 SouthLat = -33 EastLong = -70 WestLong = -72

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Hi Holger,the weekend is finally here; I have the time for the hobby and trying read the article (on http://www.elsevier.com/locate/isprsjprs ). Skipping the long complicated math formulas (definitely not for me), grateful for your simile to understand the principles. Still didn

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I think one way the holes in the 3 arc second SRTM data could be fixed is by merging LOD9 and LOD5 dataResampling the SRTM at LOD5 seems to remove most holes.So if that LOD5 could be used to "plug up" the holes in the LOD9 data then it would be much better.My question is this :-How can I use RESAMPLE.EXE to make a single .TMF which merges LOD5 and LOD9 data? Is it possible? The PDF manual seems to indicate that it is possible (they talk about the Mt Rainer example).Any ideas?Matthew.

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Uploaded v1.02 of SRTM_3ARCSEC_TO_BGL.It now interpolates to fix the missing SRTM data.So no more SRTM meshes with

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Thanks... I'll work on the 1ARC version today sometime.Matthew.

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Looks good.Actually the interpolator just derives the missing data from the average of the surrounding data values.Do you have any suggestions as to a better method of interpolation? How does MicroDEM do it... any ideas?Matthew

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Thanks... I obviously need to work on the interpolation. I'll use your tile as a reference.Matthew.

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Hi Matthew,as Holger Sandmann mentioned sooner, in the MicroDEM there is used the method Inverse Distance to power. This method is described in the Help for the Surfer ( http://www.goldensoftware.com ) as the most simple and quick to calculate. The value in the point is the weighted average of the valid points in fixed surrounding, where the weight depends on the distance, it is proportional to the inverse distance to a power, and the progress of the weight can be controlled by the value of the power (2 is default in Surfer). The sum of all weights must be one. This method creates the typical unreliable

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