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I want to use this proj4 code: +proj=utm +zone=19 +ellps=WGS84 +toclrk66=-3,142,183,0,0,0,0 +units=m +no_defs or this one +proj=utm +zone=19 +ellps=WGS84 +tonad27=-3,142,183,0,0,0,0 +units=m +no_defs

...to make on the fly transformations from WGS84 to NAD27, using the geocentric tanslation parameters recommended for Dominican Republic. I typed this codes using the "New CRS" capacity in QGIS 2.0 (Configuration>Custom CRS), but it doesn't accept the transformation parameters. After typing the codes, and pressing OK, the CRS created looks like this: +proj=utm +zone=19 +ellps=WGS84 +units=m +no_defs

QGIS deletes the geocentric traslation parameters. Maybe I'm violating proj4 codes rules, and that's why I'm asking for help.

I looked up for a solution amongst similar answers on the forum, but couldn't find anything. Sorry if I'm duplicating a question.

Any help would be appreciated.

Jose

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The layers are in WGS84, and the project CRS is NAD27 UTM 19 North.

I am not 100% sure this will work, but the convention in PROJ.4 for 3 and 7 parameter transformations is to use

+towgs84=-3,142,183,0,0,0,0

In the project properties, CRS tab, PROJ.4 text box, enter this definition:

+proj=utm +zone=19 +datum=NAD27 +ellps=clrk66 +towgs84=-3,142,183,0,0,0,0

If it doesn't appear to be working, try dropping the +datum=NAD27 option.

  • Thank you for your answer. I tried that solution, but didn't get a good result. What I'm trying to do is to transform from WGS84 to NAD27, and if I put "+towgs84" QGIS will interpret that I want to transform to WGS84. – Jose Dec 11 '13 at 17:17
  • I believe you can flip the signs and it will transform the other way. +towgs84=3,-142,-183,0,0,0,0 – Mintx Dec 11 '13 at 18:19
  • Ah, now I see. I think the layer CRS definition must include +datum=WGS84. Is the display CRS editable? That is, you choose NAD27 UTM 19 North. Can you edit its PROJ.4 string? If so, add the +wgs84 option. – mkennedy Dec 11 '13 at 18:20
  • I updated my answer and added link to a site that may help you. – mkennedy Dec 11 '13 at 18:33
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Proj does not know any +toclrk66 or +tonad27 parameters. It is always necessary to give the datum shift from a projection to WGS84, which is the universal reference ellipsoid and datum. With that proj can make the backward transformation on its own.

For 3-parameter transformations, the +towgs84 is gained from your values by just exchanging the sign, for 7-parameters, the rotation and scale values are a bit more difficult.

Alternatively, +datum=nad27 will look up all north American datum shift values from conus and other grid shift files. This overrides any +towgs84 parameter set, as long as you are inside the area of grid file definition.

  • Thanks Andre for your answer, and all of the people who helped. Now I understand that otf transformation should be done using +towgs84, because it's universal. I decided to transform only NAD27 to WGS84, and not vice versa. – Jose Dec 12 '13 at 2:27

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