# Creation of Project Datum

Needing help in creating a .prj coordinate system file that is setup for the project/local datum being used.

The coordinates are a modification of the 83/11 Washington State Plane North(M) system with an applied combined factor (divide by 0.997362 for example) with the resulting coordinate shifted by +100000 and then converted to Survey Feet.

I do not inherently understand the formatting of the .prj’s nor how to make the above formula work within it. Ideas?

Alternate formula that I think is equivalent would be 83/11 Washington State Plane North(FT) divided by 0.997362 shifted by 328083.3333

Hello eruav,
PIX4Dmapper can handle and process arbitrary coordinate systems. This includes both modified coordinate systems and fully arbitrary systems. A site Calibration file can be created to streamline the workflow and this is typically how our users process data. You can learn more about it in the link below:

If you are trying to create a custion PRJ file then you will need to use a third party software, such as ArcGIS or QGIS, to accomplish this. Either software will require the following parameters:

False Easting
False Northing
Central Meridian
Standard Parallel 1
Standard Parallel 2
Scale Factor
Latitude of Origin
Longitude of Origin

I would suggest visiting this site from Esri. It describes how to create the file using ArcGIS.

https://support.esri.com/en/technical-article/000009373

Turns out that someone had already created a .PRJ file for other software that works fine in that other software. When I use it in Mapper we are getting repetitive shifting/error of placed GCP’s that are well outside the range expected from autonomous GPS coordinates.

It would appear that mapper is not properly applying scale factors.
For example, I have a GCP with known coordinates of:
North 602806.9895 East 2823139.0340
GCP – PRJ False Northing = 274723.99
GCP – PRJ False Northing * ScaleF = 274757.93
Difference = -33.94

GCP – PRJ False Easting = 854437.03
GCP – PRJ False Easting * ScaleF = 854542.59
Difference = -105.56

These calculated differences closely match the shift/error that we see when we load the GCPs using a .CSV file while the drone points were calculated with the below .PRJ file.

Known real coords Drone Autonomous Coords Delta
603660.6505 2823216.5110 603625.749 2823107.166 -34.9015 -109.345
604710.3003 2823121.6980 604672.015 2823012.287 -38.2853 -109.411
603497.4540 2823888.2828 603462.808 2823777.068 -34.646 -111.2148
603479.8842 2822997.4290 603444.783 2822888.563 -35.1012 -108.866
603796.0641 2822999.6610 603760.073 2822890.669 -35.9911 -108.992

The file is a modification of Washington State Plane North (FT) that adds 328,083.3333’ (equivalent to 100,000m) to both northing and easting. State Plane North already has 500,000m built in as a false easting and has been scaled appropriately in this .prj with the Scale Factor prior to addition of the 100,000m.

Is there any way to confirm what I am seeing with the behavior in Mapper?

Thank You.

PROJCS[“North Spokane Corridor Project Datum CF=0.99987647”,
DATUM[“North_American_Datum_1983_HARN”,
SPHEROID[“GRS_1980”,6378137,298.257222101]],
PRIMEM[“Greenwich”,0],
UNIT[“Degree”,0.017453292519943295]],
PROJECTION[“Lambert_Conformal_Conic”],
PARAMETER[“standard_parallel_1”,47.5],
PARAMETER[“standard_parallel_2”,48.7333333333333333],
PARAMETER[“latitude_of_origin”,47],
PARAMETER[“central_meridian”,-120.8333333333333333],
PARAMETER[“false_easting”,1968702.6660],
PARAMETER[“false_northing”,328083.3333],
PARAMETER[“Scale_Factor”,1.00012354526155],
UNIT[“Foot_US”,0.30480060960121924]]

Creating a custom PRJ is a pretty advanced topic that is beyond the scope of the standard Pix4Dmapper workflow. It really would be best to consult a surveyor or geodesist as it is not very common to need to create your own PRJ. However from what you have described it sounds like the scale factor might be causing this shift. I think the best way to confirm this behavior would be to use a 3rd party GIS software like QGIS or ArcGIS. Mapper will behave the same way that these programs do. Load up the PRJ file and then add your GCPs and see where they fall.

We typically point our users to using an arbitrary coordinate system with a site calibration file. It is usually a more elegant solution to this problem. Would that work for you?