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I have a land survey document for a piece of land, it contains absolute coordinates that I would like to locate on google maps, but I don't know how to correctly interpret the data.

Coords

What does the "System: WG 21°" mean and how do I convert these coordinates to decimal notation for google maps?

The land is located in South Africa.

My Attempt:

I considered that the constants were somehow the baseline values, and tried adding the constants to each coordinate value, but the result doesn't match decimal notation. In any case, in South Africa, the latitude is negative and the longitude is positive. Both latitude and longitude are positive in this case, which makes me think that they're not latitudes or longitudes at all, but rather the result of a more complicated transform. The transform is probably indicated by the WG21 (hence the question), which is where I am currently stuck.

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As a first guess, I would pledge for Cape / Lo 21°. Would that fit? –  AndreJ Mar 5 at 11:23
    
Complete Noob here, what does Cape / Lo 21° mean? –  Stacey Mar 5 at 11:25
    
Cape is the old south african CRS datum, and Lo 21 ist the central meridfian 21°E. But the new south african datum Hartebeesthoek94 / Lo21 fits better, see my answer. –  AndreJ Mar 5 at 12:23

2 Answers 2

up vote 7 down vote accepted

I think that Hartebeesthoek94 / Lo21 EPSG:2049 fits best. I added the constant 3700000 to the X values as mentioned in the header, and imported the coordinates as delimited text into QGIS. This is what I get:

enter image description here

The coordinates are west of 21°E and south of the equator. Exporting the coordinates as WGS84 results in:

"0","20.0777332087","-33.9376197503","A","85267.05","3757125.26"
"1","20.0780827565","-33.937511304","B","85234.84","3757112.94"
"2","20.0784080105","-33.9373687785","C","85204.91","3757096.86"
"3","20.0788528052","-33.9371936109","D","85163.96","3757077.06"
"4","20.0793019671","-33.9369340943","E","85122.69","3757047.9"
"5","20.0793436801","-33.9369675812","F","85118.8","3757051.58"
"6","20.0982437624","-33.937447055","801","83370.88","3757089.25"
"7","20.0956436897","-33.9410931123","8P1","83607.71","3757495.81"

Columns 2 and 3 should give you the East and North coordinates that Google can understand.

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Well done @Andre Joost : it definitely look Hartebeesthoek94 (I didn't see your answer when writing mine) –  radouxju Mar 5 at 12:28
    
That's great, thanks! It's amazing that all it takes is just to know what to google. I googled "South Africa datum" after reading your answer and got this magical page ngi.gov.za/index.php/technical-information/geodesy-and-gps/…. Thanks :) –  Stacey Mar 5 at 12:39

The value of 21 is probably the central meridian of your local projection. There are two commonly used projected coordinate systems specific to South Africa :

Cape_Lo21 WKID: 102473 Authority: ESRI

and

Hartebeesthoek94_Lo21 WKID: 102483 Authority: ESRI

It is not possible to know which one is yours based on your information if we don't know what you are looking at.

Your coordinates are are in meters and you'll need to add the 3700000 to your Y coordinates before applying the tranformation into long/lat coordinates.

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