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scaling a tiff file for use in Global Mapper

Discussion in 'Software, CAD & Mapping' started by John Hamilton, Aug 12, 2017.

  1. John Hamilton

    John Hamilton 7-Year Member

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    I received some highway construction plans as a tiff file, with a tfw. They are "modified" state plane coordinates, having a combined scale factor applied. I want to use the tiff in Global Mapper along with imagery (which is true state plane).

    Is there any way before importing to GM to scale the tiff back to true state plane coordinates (I have the scale factor that was used)? Maybe a utility?

    Or is there an easy way to do it in GM?

    The scale factor is about 52 ppm, which in this area is about -12 feet N and -72 feet E.

    I do have a dwg file with just the alignment geometry on it, that is also in project coordinates but that is easy to scale in autocad.

    Here you can see the amount of shift:
    upload_2017-8-12_11-35-53.png
     

    Attached Files:

    Last edited: Aug 12, 2017
  2. Paul in PA

    Paul in PA 7-Year Member

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    Was the tiff created from a digital file or a paper print?

    I have scaled tiffs when bringing into AutoCAD and notice E-W/N-S scale differences from paper originals,

    Tiffs from digital originals scale properly from the tfw references as best as I can determine from tiff to ground/GPS coordinates.

    Your modified SPC may just be a translation and not a scaling process.

    Paul in PA
     
  3. John Hamilton

    John Hamilton 7-Year Member

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    PA Turnpike Mon Valley Expressway, new section constructed around 2005-2010, so I assume it was digital. And their plans specify the scale factor. I know how they do it, they multiply the scale factor times the coordinates, although ones I have done for them had the coordinates modified by chopping off a large amount so they don't look like SPC.

    Now that I think about it, shouldn't project grid (elevation is about 1200 feet) be moved to the northeast rather than southwest? Because if I multiply by a number greater than 1 (to bring it to the surface), then N and E should be larger. But the house is shifted to the SW. Or am I thinking backwards? This is very close to the southern standard parallel, so the grid scale factor is very close to 1.0 (~2 ppm).
     
  4. John Hamilton

    John Hamilton 7-Year Member

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    Here is the way I have seen it done, where only the distance from the scaling point was scaled by the scale factor. This is for a section of the Turnpike near the Allegheny River bridge. I don't like scaling at all, but this was in a control report for that section. The PTC sometimes gives us the option of true or scaled, other times they need to match previous work done. Doing as specified below results in coordinates that don't look at all like SPC but can be transformed back and forth.

    upload_2017-8-12_12-5-37.png
     
  5. Gene Kooper

    Gene Kooper 2-Year Member

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    John,

    A quick way to scale a TIFF image with a TFW file is to scale the pixel dimensions in the TFW file (1st and 4th lines). Make sure that the TIFF file is not georeferenced first (i.e. a GeoTIFF file) otherwise Global Mapper will use the georeferencing in the binary header of the image file. The TFW file is an ESRI invention. I will occasionally use Photoshop for image enhancement. When Photoshop saves a TIFF file, it will strip any georeferencing from the binary header of the GeoTIFF file.

    You can also import the GeoTIFF into AutoCAD. I use AutoCAD Map and it tells me the source of the georeferencing (e.g. whether it is using the TFW file and whether I would like to change the source of the georeferencing). You can scale any image using the Scale command. You can also check if the image is georeferenced by removing the TFW file from the directory you have the image in. If Global Mapper asks for georeferencing before loading the image, the image doesn't have georeferencing in its header.

    The problem I see with getting the coordinates in SPCs is determining the false northing and easting of the origin point that was used to scale and translate the image to ground and the modifrickinfied SPCs coordinates. Good luck with that if you don't have the metadata.
     
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  6. Kent McMillan

    Kent McMillan Banned

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    Well, one method that comes to mind would be to to

    - scale the DWG file to the SPCS,
    - import scaled DWG into GM,
    - import just TIFF image and rectify using common points in DWG file.
    - export TIFF with new TFW for SPCS coordinates
     
  7. Kent McMillan

    Kent McMillan Banned

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    Another more direct method would be to just load the tiff with tfw and in CONTROL CENTER highlight the layer and right click to bring up a menu with SHIFT as the heading within which shifts and transformations are found. I'd still have the DWG loaded on top of it to verify that you weren't ending up with something that was obviously inconsistent.

    Then, when the tiff layer is shifted and transformed, export it as a tiff with a new tfw and import that instead of relying upon the GM workspace to transform the funny tiff.

    NB: The above are for GM V15.
     
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  8. FrozenNorth

    FrozenNorth 2-Year Member

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  9. Paul in PA

    Paul in PA 7-Year Member

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    I would bring it in and scale only for grid and not elevation. I would assume the flat plan is at the zero elevation, while the orthophoto surface was at elevation and already brought down to zero before distribution. Get a good look at it and then scale for elevation as a separate step if necessary and review again.

    The elevation scale factor for 1200' is below 1 and brings the scaled drawing closer to the base point and not false coordinates zero.

    Paul in PA
     
  10. R.J. Schneider

    R.J. Schneider 7-Year Member

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    There was a way to change the geo-reference in the world file also. It's been over a decade since i've done this but, if I remember correctly, the coordinates in the world file placed the center of the top left image pixel. I had done this with the earlier TNRSS 1M doqqs.
     
  11. John Hamilton

    John Hamilton 7-Year Member

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    So I guess I wasn't given all of the information...

    we are going to scan a ~1000' section of highway to find and map a "dip" in the pavement. This is the fourth area we have done this for. They really like having the dense point cloud to be able to see exactly where the dip is located and to plan a remedy. It may be due to subsidence, undermining, who knows?

    I entered the alignment data (mainline and parallel on/off ramps) into CAD. This particular page of alignment data states "Project Coordinates" at the bottom. I exported a kmz file, and brought it in to GE. It overlays PERFECTLY (at the scale of the imagery, which is 2016). So I believe they must have scaled about a nearby point instead of at the origin as i had assumed. I just don't have that information, but I have requested it. The only thing the set of plans i have says is that to get from SPC distance to ground distance, multiply by X.xxxxxxxxxxx, it does not really explain it any more than that.
     

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