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I've been attempting to extract track information from Humminbird (Side Imaging Sonar) binary files and have hit a stumbling block with the coordinates it provides. The values are apparently in Mercator meters but no UTM zone is supplied in the output. I would like to know if anyone knows of a mercator meter conversion that doesn't require a UTM zone, and if so how do you do it? I've found one conversion which kind of does the job but not acurately enough (http://www.oziexplorer3.com/eng/eagle.html).

So basically I get the numbers Lon: 16226043 Lat: -4557743. I know these should convert to 145.75528834210039 and -38.032398555522214 but can't work out how.

I've had a discussion about this issue on a more dedicated forum but came to a dead end (http://forums.sideimagingsoft.com/index.php?topic=5623.msg35417). I could be reading the binary section wrong but all other data in the line comes out fine.

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Thanks to your good description I guessed that the projection that Hummingbird is using is "WGS 84 / World Mercator" which has EPSG-code EPSG:3395. More information about this projection:

http://spatialreference.org/ref/epsg/3395/

http://epsg.io/3395

http://epsg-registry.org/report.htm?type=selection&entity=urn:ogc:def:crs:EPSG::3395&reportDetail=short&style=urn:uuid:report-style:default-with-code&style_name=OGP%20Default%20With%20Code&title=EPSG:3395

For converting coordinates between different projections you can use for example gdaltransform http://www.gdal.org/gdaltransform.html

Test with your coordinates:

gdaltransform -s_srs epsg:3395 -t_srs epsg:4236
16226043 -4557743
145.751771880036 -38.033447580973 -512.892291001976

Latitude and longitude seems to be accurate. I do not know what -512 as Z-coordinate means but perhaps it has something to do with the WGS84 spheroid.

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