I'm working on a project where I'd like to define a projection in QGIS, using a Proj4 string, so that a user can import and export site coordinates from the local UTM or geographic coordinates.
I've had a good search around the place, and the best resource I found is this gis.stackexchange post, which gets me close, but I have errors still, which you can see in the images below. They are not systematic in a way that I can see, but might be related to scale.
This is my Proj4 string:
+proj=omerc +lat_0=-42.83254553 +lonc=147.31794080 +alpha=43.297962 +k_0=0.999592 +x_0=0 +y_0=0 +gamma=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
And I have used a bit of software to generate transformation parameters from known coordinate relationships, which results in these:
- X shift: 525985.101348
- Y shift: 5257731.440819
- Z rotation: 0.755692
- Scale: 0.999592
These were what I used to get the parameters in the Proj4 string, including coming up with the origin Lon and Lat (via transforming the 0,0 point back to MGA and thence forward to WGS84).
If I use the above parameters to do a manual transformation, the coordinates are transformed perfectly. If I use the above projection in QGIS I'm still getting errors of up to 2 m.
I wonder if anyone knows of a better projection string to use in Proj4, or if there is another way that can handle a site datum or custom coordinate system with a transform into a known system.
EDIT: more info
Here's some coordinate pairs. Note that the lat/lon are derived from the MGA coordinates. E and N mga is GDA94, MGA Zone 55. The Zs are irrelevant it's a simple vertical shift.
Note that I worked out the Lat/Lon shift for the Proj4 string by converting a 0,0 site coordinate to MGA, and used QGIS to convert this to WGS Lat/Lon. This give the origin Lat, Lon as per the other stackexchange post.
Emga,Nmga,Eplant,Nplant,Lon,Lat,Zmga,Zplant 525631.34,5258107.55,-515.61,31.13,147.3135953,-42.82917059,10.7,10.02 525634.87,5258111.13,-515.49,36.17,147.3136383,-42.82913824,10.33,9.65 525633.12,5258109.03,-515.33,33.44,147.313617,-42.82915721,12.43,11.74 525633.98,5258109.93,-515.32,34.68,147.3136275,-42.82914907,9.76,9.08 525692.84,5258084.4,-454.95,56.47,147.3143487,-42.829377,3.09,2.4 525692.41,5258083.45,-454.61,55.49,147.3143435,-42.82938557,3.14,2.46 525856.42,5258016.48,-289.24,119.25,147.3163532,-42.82998312,3.2,2.52 525876.22,5257998.88,-262.762,120.021,147.3165963,-42.83014095,9.59,8.91 525907.25,5258001.29,-241.82,143.07,147.3169758,-42.83011819,3.99,3.3 525906.52,5257997.55,-239.79,139.84,147.3169671,-42.8301519,3.23,2.54 525912.12,5257997.14,-235.43,143.38,147.3170356,-42.8301554,3.17,2.49 525925.34,5257969.74,-207,132.5,147.3171986,-42.8304017,3.41,2.73 525939,5257962.35,-191.98,136.49,147.3173661,-42.83046778,2.89,2.21 525934.89,5257911.72,-160.24,96.81,147.3173181,-42.83092385,2.91,2.23 525933.72,5257861.72,-126.8,59.6,147.3173061,-42.83137416,2.91,2.23 525981.79,5257875.04,-100.93,102.28,147.3178936,-42.83125257,41.97,41.29 526006.26,5257860.79,-73.34,108.7,147.3181937,-42.83138007,8.59,7.9 526086.76,5257842.88,-2.44,150.89,147.3191794,-42.83153861,8.79,8.1 525788.67,5257525.66,-1.84,-284.59,147.3155468,-42.83440535,10.52,9.83 525788.87,5257456.4,45.82,-334.89,147.3155524,-42.83502905,10.47,9.79 526149.54,5257795.97,75.46,159.8,147.3199497,-42.8319589,10.48,9.8 525789.24,5257402.47,83.09,-373.9,147.3155594,-42.83551469,42.49,41.81 526184.03,5257761.77,124.03,158.57,147.3203733,-42.8322657,37.11,36.43