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"Shift in position?"

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chrisnoble (6 posts)
Feb-19-02, 03:50 PM (EST) "Shift in position?" I have just purchased DEMEX V1.0.As an experiment I loaded a sample file DTMSS68.NTF from the Ordnance Survey site in the UK. This is for the Swansea area in South Wales (Swansea is roughly 51.37N, 3.59W). I have loaded this into Microdem and the cursor indicates the correct position. I save this as a Microdem DEM file.When I open this in DEMEX V1.0 the position is Lat 1.75 Long -119.09. What am I doing wrong? Alert Edit | Reply | Reply With Quote | Top
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Subject Author Message Date ID RE: Shift in position? StHarlan Feb-19-02 1 RE: Shift in position? JohnCS Feb-19-02 2 RE: Shift in position? pguth Feb-19-02 3 RE: Shift in position? chrisnoble Feb-28-02 15 RE: Shift in position? JohnCS Feb-28-02 16 RE: Shift in position? chrisnoble Feb-20-02 5 RE: Shift in position? chrisnoble Feb-20-02 6 RE: Shift in position? Puffing_Billy Feb-20-02 4 RE: Shift in position? chrisnoble Feb-20-02 7 RE: Shift in position? Puffing_Billy Feb-21-02 10 RE: Shift in position? JohnCS Feb-20-02 9 RE: Shift in position? Puffing_Billy Feb-21-02 11 RE: Shift in position? LawrenceKellie Feb-21-02 12 RE: Shift in position? JohnCS Feb-20-02 8 RE: Shift in position? chrisnoble Feb-22-02 13 RE: Shift in position? LawrenceKellie Feb-22-02 14

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StHarlan (6 posts)
Feb-19-02, 09:37 PM (EST) 1. "RE: Shift in position?" Sounds like you should contact the author of DEMEX directly.http://www.demex.vcn.com/ Remove | Alert Edit | Reply | Reply With Quote | Top
JohnCS (303 posts)
Feb-19-02, 09:38 PM (EST) 2. "RE: Shift in position?" I am not familiar with NTF files, however here are a couple of options and solutions depending on how you translated the file in Microdem. (This is based upon the assumption that NTF files are latitude/longitude based instead of UTM):a) You resampled the file in microdem into UTM coordinates. Until lately Microdem did not convert some european zones correctly. Make sure you download the latest 'debug' version of Microdem from the Microdem site (you should have a version dated at least 1/31/2002). You can check this by reloading the dem into microdem and seeing what the coordinates read when reloaded.Alternatively just save it as a DEM without resampling, which will save it in lat/long format and try loading that dem in DEMEX.b) You are loading it in DEMEX in lat/long format and there is probably a DEMEX bug. In this case please point me towards a source for a NTF file (please do NOT email one to me) and I'll try to duplicate the problem. Remove | Alert Edit | Reply | Reply With Quote | Top
pguth (12 posts)
Feb-19-02, 10:18 PM (EST) 3. "RE: Shift in position?" NTF uses the OS grid, which is kind of like UTM, but all the parameters are different. It's treated as a special case in MICRODEM.If another program can't deal with the OS grid, I'd suggest converting to DTED in MICRODEM, and then converting that to UTM or a standard MICRODEM DEM, which should work. Remove | Alert Edit | Reply | Reply With Quote | Top
chrisnoble (6 posts)
Feb-28-02, 06:17 PM (EST) 15. "RE: Shift in position?" I previously blamed MSTS RGE for misrepresenting the position. I have done some further work with other ntf tiles from a different location in the UK and I have found that Microdem appears to be translating the data from its true position. The corner of tile SP04 is N52d 3.517' W2d 0.000' from an Ordnance Survey Map but Microdem reports this as N56d 16.302' W3d 36.887' and this remains constant even with a change of format. I hope that Dr Guth can throw some light on this.I could follow the suggestion of Puffing Billy but I would like to avoid fudges if possible.Incidently Academics and students at some universities can get free access to UK OS data through the Digimap scheme provided that it is for research or educational purpose Remove | Alert Edit | Reply | Reply With Quote | Top
JohnCS (303 posts)
Feb-28-02, 07:50 PM (EST) 16. "RE: Shift in position?" Two suggestions:1) The best place to get Microdem help is in the Microdem forums. There is a link on the help menu of Microdem.2) Before you get too excited double check the datum from your maps against the default Microdem datum. Microdem will shift the datum to WGS84, which is probably different than your topomaps (it will say on the topomap somewhere what the datum is). This means that the lat/long coordinates of any given point will shift (though you shift sounds extreme). You can change the default datum in Microdem to a different datum to adjust the coordinate display.
-JohnCS
DEMEX
http://www.demex.vcn.com Remove | Alert Edit | Reply | Reply With Quote | Top
chrisnoble (6 posts)
Feb-20-02, 02:15 PM (EST) 5. "RE: Shift in position?" The URL is
http://www.ordnancesurvey.co.uk/
The product is Land-Form Panorama under Business geographic
the sample is at the bottom of the product details page Remove | Alert Edit | Reply | Reply With Quote | Top
chrisnoble (6 posts)
Feb-20-02, 02:16 PM (EST) 6. "RE: Shift in position?" The URL is
http://www.ordnancesurvey.co.uk/
The product is Land-Form Panorama under Business geographic
the sample is at the bottom of the product details page Remove | Alert Edit | Reply | Reply With Quote | Top
Puffing_Billy (6 posts)
Feb-20-02, 06:25 AM (EST) 4. "RE: Shift in position?" Chris,While the other respondents are correct, I am not certain that the samples from the OS site have not been doctored slightly.
The ss68 tile does not have the correct reference point when it is imported into Microdem. Here's what I did to correct it.1) Open the NTF file in Microdem. Immediately save it as a native
Microdem DEM.2) Open the Microdem Data Manipulator ( the In/Out button ).
Choose Edit >> headerSet the header information as x = -14370 y = 187215 .
That is the UTM coordinates for the South West corner.Note that this is just an estimate. It is difficult to guess the actual point when it is in the water. To get a true value you will need to obtain the OS map which contains ss68, get the lat and long for the SW corner, convert it to UTM with Microdem.Anyway, the values given are probably close enough.3) Convert the file with the edited header to DT0 - which ensures that it properly conforms to the WGS84 datum - and arranges the coordinates in the right order for Demex.4) Convert the DT0 file to XYZ and use with Demex.
Hope this helps - it worked for me.RegardsWill Remove | Alert Edit | Reply | Reply With Quote | Top
chrisnoble (6 posts)
Feb-20-02, 05:52 PM (EST) 7. "RE: Shift in position?" Thanks I will try this tomorrow when I get chance. Is this the case for all Ordnance Survey files?
I was considering buying some but they are £10 for a 20Km tile and the minimum order is now £100 + VAT.
The Americans are lucky having free access to their data.
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Puffing_Billy (6 posts)
Feb-21-02, 05:04 AM (EST) 10. "RE: Shift in position?" >>Is this the case for all Ordnance Survey files? I am not sure, I haven't tried to purchase any others yet.
>I was considering buying some but they are £10 for a 20Km tile and the minimum order is now £100 + VAT. Agreed, the price is a barrier. But when you consider that you can terra form each mile of track for a pound, it doesn't seem that bad compared with following contours on a map.In addition it makes sense to purchase the corresponding
Landranger data for the same grid squares. This lets you create markers to the same scale at the terraform data. It is also ten pounds per 20km grid square.You can of course get this map data free from the Internet, but it is an extremely tedious process getting the data in very small chunks and stitching it together to make a half decent sized map.
>>The Americans are lucky having free access to their data.
Agreed. Remove | Alert Edit | Reply | Reply With Quote | Top
JohnCS (303 posts)
Feb-20-02, 08:10 PM (EST) 9. "RE: Shift in position?" Never willingly use XYZ coordinates with DEMEX V1.0, but go with the native or Microdem dem format. XYZ was only included for compatibility with existing datasets, or as a generic input method for unsupported formats. Remove | Alert Edit | Reply | Reply With Quote | Top
Puffing_Billy (6 posts)
Feb-21-02, 05:08 AM (EST) 11. "RE: Shift in position?" John,I hear what you are saying, but in version 0.6 the only way that British data could be inserted with Demex was to go through theDEM >> DT0 >> ASCII XYZ UTM processIt was the only way that the data would be ordered in the correct way AND converted to WGS84.I really must try it with Demex 1.0 to see if Microdem DEMs work as you designed it with UK data.And someday soon I really ought to buy the program ( now that it is officially released). Remove | Alert Edit | Reply | Reply With Quote | Top
LawrenceKellie (1431 posts)
Feb-21-02, 11:26 AM (EST) 12. "RE: Shift in position?" Dr. Guth, the creator of MicroDem, has made a lot of changes to MicroDem since DEMEX version 0.6 days, and many of those fixes have been to make MicroDem more useful to the MSTS community.I have no idea whether or not this particular situation has been changed, but it certainly would be worthwhile to get the latest version of MicroDem, which, by the way, will not overwrite the version 5.x copy you may be using.Lawrence Remove | Alert Edit | Reply | Reply With Quote | Top
JohnCS (303 posts)
Feb-20-02, 08:08 PM (EST) 8. "RE: Shift in position?" Dr. Guth in post 3 is correct. (Thank you very much, I would have had great difficulty narrowing this down without the OS grid clarification).Note that reloading your saved MD DEM file into Microdem will produce the same shifted results. The following steps worked for me using the 1/31/2002 version of Microdem:1) Load the original NTF into Microdem, and save it as a DTED file. I choose intervals of 1 arcsecond since that is slightly less thant the 50 meter grid spacing in the sample file. The file will save with the DEM extention, however this is still not a file Demex can read.2) Close your original file in Microdem, then load the DTED DEM file you created. Now save this file using MD DEM format.3) You can now load this DEM file into DEMEX. Remove | Alert Edit | Reply | Reply With Quote | Top
chrisnoble (6 posts)
Feb-22-02, 01:38 PM (EST) 13. "RE: Shift in position?" Thanks to you and Dr Guth for your help. It worked. The location in RGE seems to be out. According to RGE Swansea is in the middle of the Bristol Channel about 26 miles away from its real position Remove | Alert Edit | Reply | Reply With Quote | Top
LawrenceKellie (1431 posts)
Feb-22-02, 01:55 PM (EST) 14. "RE: Shift in position?" Wow! Gives new meaning to "How long can you tread water?" Lawrence Remove | Alert Edit | Reply | Reply With Quote | Top

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