<div dir="ltr"><div class="gmail_default" style="font-size:small">There was a long discussion in this mailing list back in 2017 about 7 parameter transformations with large rotations. You can find the discussion at <a href="http://osgeo-org.1560.x6.nabble.com/towgs84-approximation-error-td5313738.html">http://osgeo-org.1560.x6.nabble.com/towgs84-approximation-error-td5313738.html</a>.</div><div class="gmail_default" style="font-size:small"><br></div><div class="gmail_default" style="font-size:small">I'm not sure that there was ever a resolution to the discussion. But if PROJ is still using small-angle approximations for 7 parameter transformations then that is probably the reason that it isn't giving the expected results?</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, 2 Aug 2019 at 03:39, Michel Ruijter <<a href="mailto:mnrengineering@gmail.com">mnrengineering@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_default" style="font-family:arial,helvetica,sans-serif">Ls,<br>For a small Caribbean Island (Saba) I am trying to get the proper proj4 CRS (for Qgis) in order to use GPS based coordinates on local grid related system<br>The following projection and datum parameters are used in a TRimble software package (and used in the RTK equipment) in local "Kadaster" coordinates to set out benchmarks etc<br><br>Ellipsoid<br>Ellipsoid Name : (WGS 84) <br>Flattening 1/f : 297.000 <br>Semi Major Axis : 6378388.000m <br><br><br>Datum Transformation : Seven Parameter<br>WGS84 to (WGS 84) (sounds a little bit confusing since ellipsoid seems to be the international ellipsoid<br>Translation X : 194.245m Rotation X : 0°05'51.504629" <br>Translation Y : -154.530m Rotation Y : -0°11'41.841439"<br>Translation Z : 102.676m Rotation Z : 0°04'11.988863"<br>Scale Factor : 6.2965571264 ppm<br><br><br>Transverse Mercator Projection<br>Projection Origin False Origin<br>Latitude : 17°38'10.36749"NFalse Northing : 1967.190m<br>Longitude : 63°14'15.07247"WFalse Easting : 4714.870m<br>Height : N/A False Elevation : N/A<br>Scale Factor : 1.00000000<br><br>Test coordinates: <br> WGS84 in°N WGS84 in°E z(ellips) LocalE in m LocalN in m Z(msl)<br>1 17.61578594 -63.25156367 -41.19 3215.38 -287.59 2.065<br>2 17.62323214 -63.23827421 290.27 4629.098 530.823 333.55<br><br>These coordinates have been verified with pctrans (software of the dutch navy) and the conversion (Trimble) seems to be good to within 2 cm<br><br>I have tried to convert the translation rotation and projection into a proj4 structure within Qgis<br>I converted the rotation into arcseconds:<br><br>Rotation X : 0°05'51.504629" (351.504629")<br>Rotation Y : -0°11'41.841439" (-701.841439")<br>Rotation Z : 0°04'11.988863" (251.988863")<br><br>+proj=tmerc +lat_0=17.63621319 +lon_0=-63.23752013 +k=1 +x_0=4714.870 +y_0= 1967.19 +ellps=intl +towgs84=194.245,-154.53,102.676,-351.5046,701.841,-251.9889,6.296557 +units=m +no_defs<br><br>(note all signs of of rotation have been changed compared to Trimble notation )<br><br>application of this proj4 string in Qgis results in the following two local grid positions:<br>1 3250.9 mE -209.59mN<br>2 4658.157mE 620.26 mN <br><br>I also tried to use OS4GEO installed on a win10 machine in many variants e.g.</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif"><br>echo -63.25156367 17.61578594 |cs2cs +proj=latlong +ellps=WGS84 +towgs84=0,0,0 +no_defs +to +proj=tmerc +ellps=WGS84 +lat_0=17.63621319 +lon_0=-63.23752013 +k=1 +x_0=4714.87 +y_0=1967.19 +towgs84=-194.245,154.53,-102.676,-351.5046,701.841,-251.9889,-6.296557 +units=m +no_defs<br><br> with as a result x=3458.54 y=-202.09 z=286.19</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif"><br></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif">I have tried to use the pipeline option as well however, in one of the first steps OSgeo seems not to convert coordinates properly:</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif">e.g. with the statement below I expect to get 17.00 17.00 as an answer but I get 17.00 0.00 !!!</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif"> </div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif">echo 17.0 17.0 | proj +proj=pipeline +step +proj=cart +ellps=intl +step +proj=cart +inv +ellps=intl<br></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif"><br><br>I have tried a number of versions of this transformation (changing signs in the towgs string string but this does not result in proper coordinates.<br><br>What is wrong with this proj4 string based on the rotation and projection information from the Trimble software and how in general can we use the translation and projection information into a valid proj4 string?<br><br>Kind regards,<br><br>Michel Ruijter<br clear="all"></div><div><br></div>-- <br><div dir="ltr" class="gmail-m_5149735053632066405gmail_signature"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr">Michel Ruijter<div>MNRengineering</div><div>de Kempenaerstraat 82, 2341GP, Oegstgeest</div><div>mob: +31638944484, tel: +31718897137</div></div></div></div></div></div></div></div><div id="gmail-m_5149735053632066405DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2"><br> <table style="border-top:1px solid rgb(211,212,222)">
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