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what datum to use for USGS topo maps?

 
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erdos



Joined: 30 Jul 2014
Posts: 18

PostPosted: Wed Aug 10, 2016 10:03 pm    Post subject: what datum to use for USGS topo maps? Reply with quote

I created my local maps with 'mapc2mapc' program using USGS top maps in geotiff format. they display ok but there's an offset comparing with the actual location.

after some online search, I discovered the datum might have something to do with it. so I've tried to set the datum on TB to 'WGS84' and 'NAD27', but the offset still exist.

any suggestion on how to fix the offset?
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Ranger



Joined: 12 Dec 2006
Posts: 6027

PostPosted: Fri Aug 12, 2016 12:16 pm    Post subject: Reply with quote

Map, to be supported well in app, should have correct calibration file.
Datum set option in TB is used only when calibration file datum isn't recognized by app, probably not your case.
Could you attach map calibration file (prefered) or just list content ? Smile Kind of used tool is less important.
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erdos



Joined: 30 Jul 2014
Posts: 18

PostPosted: Fri Aug 12, 2016 11:00 pm    Post subject: Reply with quote

thanks for the repliy!

there are two files along with .set folder generated when i converted the geotiff map to TB format.

please see attached files compressed into .zip format.



DC_Washington West_255913_1986_100000_geo_tiles.zip
 Description:

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 Filename:  DC_Washington West_255913_1986_100000_geo_tiles.zip
 Filesize:  2.21 KB
 Downloaded:  146 Time(s)

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Ranger



Joined: 12 Dec 2006
Posts: 6027

PostPosted: Sat Aug 13, 2016 10:29 am    Post subject: Reply with quote

Map is calibrated in WGS84 and TM projection with scale 1.0. Seems rather unusual for USGS.
Second - map is dated 1986, means comes probably from historical maps series.
Did you tried to find GeoPDF version, to be sure about original datum and projection ? Smile
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erdos



Joined: 30 Jul 2014
Posts: 18

PostPosted: Sat Aug 13, 2016 2:07 pm    Post subject: Reply with quote

thanks, problem solved!

there are four files included when i unzip the tiff package downloaded from USGS website:
tif file
tfw file
xml file
PRJ file

i looked into each file and found the following in .xml file:
<geodetic>
<horizdn>D_North_American_1927</horizdn>
<ellips>Clarke 1866</ellips>
<semiaxis>6378206.4</semiaxis>
<denflat>294.9786982</denflat>
</geodetic>

it seems to indicate that the map is using 'NAD27' datum, so i specified NAD27 when i covert tiff map to TB format, and the test result:
offset is gone, the map aligns with actual locations.

i recalled first time around, i was prompted by the program on what datum to use for opening the tiff, i chose 'WGS84' instead of 'NAD27', hence caused the offset.

as you mentioned earlier, in TB, datum is determined by set file. i found changing datum settings in TB has no effect on the map alignment.

on a side note: USGS maps seem to be using a mix bag of 'NAD27', 'NAD83', and 'WGS84' datums.

Thanks for the help!



DC_Washington West_255913_1986_100000_geo.map
 Description:

Download
 Filename:  DC_Washington West_255913_1986_100000_geo.map
 Filesize:  3.21 KB
 Downloaded:  131 Time(s)

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