# GCP conversion for South Africa

Okay here I go,

First, I’m new to drone mapping so I’ll try my best with the terminology, second English is not my first language so bear with me

So I have captured an area with GCP points on the ground. So I have the basics covered there, I think.

I have coordinates for the GCP points but in the Ground Control Point Interface I loose the plot with the conversion from the ‘South Africa Lo System’ to the UTM system

I know the UTM where I need to convert to is, WGS84 UTM 36S

So my text file looks like this before I import it:

WGS84 UTM 36S
GCP1 87069.548 2757317.034 1224.312
GCP2 87136.539 2757239.324 1216.892
GCP3 86991.957 2757143.721 1232.984
GCP4 87054.578 2757109.373 1241.562

(I only have 4 points for now)(I would like to at least plot these 4 points on the correct position and on the next map I will have 5+)

opendronemap’s Ground Control Point Interface does pick up there are coordinates in the text file and plots the 4 points on the map, but on the wrong location.

I have converted the points to decimal degrees but no success, I am totally lost at the moment.

Any help would be much appreciated.

Thank you for the awesome community and forum. I have gained tremendous insight from here already…

Some screenshots
Image 1:

Carlleo,
Welcome to the community.
I think the first line of your control point file should specify the coordinate system that your control points are expressed in. In your case, I think ( and you should verify) it is the following proj4 string:

+proj=tmerc +lat_0=0 +lon_0=33 +k=1 +x_0=0 +y_0=0 +axis=wsu +a=6378249.145 +b=6356514.966398753 +towgs84=-136,-108,-292,0,0,0,0 +units=m +no_defs

I believe when you import the control point file into the gcp interface it exports the correct utm gcps in the gcp_file.txt. Let me know if this works.
Mike

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