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I have some data (points and rasters) that I would like to be able to convert to and from several vertical datums, including MLLW in coastal Florida.

Can you provide some insight into accuracy, pitfalls, requirements, etc. of using available utilities such as VDatum or corpscon?

Is one more user-friendly than the other?

Are there any others that I should consider?

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4 Answers 4

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You might also consider GDAL via OSGeo4W, which has the ability to do vertical datum transformations. I don't think that the OSGeo4W build has grid files for coastal Florida, but they could be developed. Search the GDAL Trac wiki or ask on the gdal-dev list for more information on how to develop your own .gtx grid file for coastal Florida.

GDAL, of course, has the advantage of being much easier to use than VERTCON, VDatum, or corpscon with the ability to open just about any vector or raster format.

A local thread with a couple recipes: Vertical Datum conversion of a raster with gdal

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If you are doing vertical conversions over the ocean you really need to use VDatum.

Vdatum runs a detailed, unstructured mesh tidal model (ADCIRC model, usually), then computes tidal coefficients, generates an 18.6 year time series, then determines surfaces of MLW, MLLW, MHL, MHHW, etc. These surfaces are then evaluated at known tidal gauge locations, and the deviations are used to fit another surface which is then used to adjust the grids so there is no error at the gauge locations.

In the final step they intepolate to a regular grid, which forms the basis so the software conversion. But you can ask for the original surfaces on the original mesh (usually triangular) as well. I did this when I was correcting datums in the Gulf of Maine.

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In Canada, see GPS-H from Canadian Geodetic Survey. As the name implies it's suitable for point records and not lines & polygons, and unlikely a good fit for mass points. Input formats are ascii text files like CSV.

GPS-H allows conversion of GNSS ellipsoidal heights h, that are either in NAD83(CSRS) or ITRF, to orthometric heights H (heights above mean sea level) through the application of a gravimetric or hybrid geoid model. ... also the reversed conversion (H to h), and ... between two vertical datums (Ha to Hb) (e.g. CGVD28 to CGVD2013). ... orthometric heights are directly integrated into CGVD2013. ... accepts different types of coordinates: geographic (D.d, DM.m and DMS.s), Cartesian (X, Y, Z) and local projection systems (UTM, MTM, stereographic and custom projections).

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I loved Corpscon when I did a lot of datum conversion because it was stand alone and I'm not a programmer.

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