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Easting / Northing help
Posted by quigley595 on February 24, 2019 at 10:52 amI am a complete newbie, and have been struggling with this issue for a while.
Apologies if this is the wrong forum, but I have a question on Easting , Northing which I hope you can help me with.
I am in Australia, and have a project description which states:
MGA Coordinates: E: 4,96623.669 N: 6,978,669.504 RL 3.268 AHD
Project Coordinates: E: 2,000.00 N: 5,000.00 RL 3.268 AHD
The MGA Coordinates are for the project datum… I know the associated Lat Long for the MGA are: 27.315044, 152.965874
Question now is…. if I have a Lat/Long further into the project area, and I convert these to Easting/Northing (which I can do)…. what do I do with the ‘Project Coordinates’ of 2,000 and 5,000…..
Do I add these, subtract them, or do nothing in regards to my calculated Easting/Northing figures?
thanks for any help on this one,
Mike
quigley595 replied 5 years, 7 months ago 9 Members · 17 Replies 
17 Replies

It’s not a matter of adding or subtracting. You need to do a transformation from local to geocentric or vice versa.

I believe MGA is similar to UTM but adapted to your national datum.
Subtracting local at a reference point from MGA at the same point to get a conversion offset and then subtracting that offset from other MGA values to convert them to project coords will work for a small area. The acceptable size depends on the accuracy required. I’m guessing it might work for a city block but not for a large farm, but that’s only a guess.
As you move further from the reference point the scale factor of the MGA projection needs to be taken into account to keep your lengths matching ground.
There is also the matter of bearings. MGA bearings will not match geodetic bearings by some mapping angle, and this offset changes as you move around.
. 
You might find this link of interest.
http://spatialservices.finance.nsw.gov.au/surveying/geodesy/gda/mga_combined_scale_factor
. 
Often, when running simultaneous coord systems, you have lat Lon, and your spc coords, which with a defined projection, and parameters, are reciprocals of each other. So, for practical use, they are enough the “same”, to call them “unity”, or the same.
Now, as to those pesky little 2k,5k coords.
Usually that’s a local coord system.
PT # (spc) = pt # (local)
Combined scale factor (csf) = (and this number is slightly more than one, or slightly less than one)
Rotation factor is either zero, or theta, (that would be the degreesminutesseconds difference between geodetic and spc) or whatever local system desired. Usually not more than a few degrees of rotation.
Then, sometimes, if elevations are critical, and the geoid is not flat, then a tilt, in NS, or EW, or a combination of those.
TDS software did several bad things to the world of surveying. It HID some of these parameters, and many users used “localize” without comprehending what was going on.
Some surveyors today still use “localize”, but are in the dark as to the mechanics, and details of this common multiple parameter action.
I know what I’ve posted won’t fully satisfy your need to know, but it will provide you an outline for what is going on.
Nate

There isn’t much that can be done with data on one point.
All that can be done with the information given is a bunch of guessing.

You could build a transformation set holding those points as common and zero scale and rotation. Follow up by locating project control at the limits of your work area. Check the scale and rotation of your shots to see if it matches a computed combined factor and any typical angle differences of differing ‘north’.
Or you can call the person publishing the plans and ask…

You need more than just a single coordinate pair. Either more coordinate pairs in the the same local system or scale factor and rotation.

Looking at this link, making assumptions about offsets and rotations might be dangerous. Somewhere there are likely some definitions that will make it all very clear.

Posted by: MightyMoe
There isn’t much that can be done with data on one point.
All that can be done with the information given is a bunch of guessing.
Thanks Moe,
yes, I realise I cant do anything with one point….. I was just trying to get the concept….
I do of course have many points, in fact, thousands of them… I am trying to work on a Point Cloud file in XYZ format, and convert this to E/N format (which I can do), but the Project Coords were throwing me a little…. I didn’t know if I should incorporate these (add / subtract or whatever).
Then there is the Scale factor….. 0.99958271 for Ground to Grid, and 1.00040189 for Grid to Ground…… I didn’t mention these in my OP… and not sure if Ishould factor them in to my E/N conversion
rgds
Mike

Posted by: Dave Karoly
You need more than just a single coordinate pair. Either more coordinate pairs in the the same local system or scale factor and rotation.
Thanks Dave….. see my reply to MightyMoe…
rgds
Mike

Thank you so much to all for your fantastic replies….. you have been excellent!!!!
I am still coming to grips with all of this, and you all have been a great benefit in pushing me further along that road.
regards
Mike

Which points are common to both files?
Are there pt descriptions?
Can you isolate the control points, in both files?
Just guessing here, but if there are enough points common, this should not be too hard… Btw, how many points are there in the spc file? And in the local file?
I mean, look for points that have common names, and that there are not many of… This could help.
Nate

Posted by: quigley595Posted by: MightyMoe
There isn’t much that can be done with data on one point.
All that can be done with the information given is a bunch of guessing.
Thanks Moe,
yes, I realise I cant do anything with one point….. I was just trying to get the concept….
I do of course have many points, in fact, thousands of them… I am trying to work on a Point Cloud file in XYZ format, and convert this to E/N format (which I can do), but the Project Coords were throwing me a little…. I didn’t know if I should incorporate these (add / subtract or whatever).
Then there is the Scale factor….. 0.99958271 for Ground to Grid, and 1.00040189 for Grid to Ground…… I didn’t mention these in my OP… and not sure if Ishould factor them in to my E/N conversion
rgds
Mike
That is a large scale factor.
Inverse out a coordinate pair and see if the inverse between the 2000, 5000 pair matches with an inverse of the same pair of MGA coordinates, either in distance and or azimuth. If the distance is different by the 1.0004 scale factor you will know that the 2000,5000 pair have been adjusted to ground, if the azimuth matches you will know that the 2000,5000 coordinate pair is using the MGA grid azimuth. If the distance and azimuth match then someone has applied a simple shift in Easting, Northings. If the distances either to the MGA system or the 1.0004 scale match but the azimuth is off maybe someone tried to create a “true north” little grid system. Once you have a few coordinate pairs it can be puzzled out.

Thanks Moe,
heres the thing… the surveyor gave me a document with the MGA & Project coordinates as in my OP, and also the scale factors as in my quoted post to you, just above…. Unfortunately she can’t be contacted right now for me to get some insight from her.
The Point Cloud I have was taken from a UAV (drone), and was in Lat / Long, which I converted to UTM E/N…
My task is to ensure that these E/N conversions are correct, and not being a surveyor myself, I wasn’t sure about applying the 2,000 and 5,000 datum (I don’t think I should, since this would throw things out by a lot, the 2,000 and 5,000 being metres – the site isn’t that big.
So I guess I either leave the Point Cloud converted figures as they are, or apply the scale factor, but I’m not sure if it’s Ground to Grid, or Grid to Ground.
I will play around with a few points and the inverse as you suggested.
rgds
Mike

The Point Cloud I have was taken from a UAV (drone), and was in Lat / Long, …
Huh? Wuh?!?! How is that even possible?

Now we’re getting the “rest o’ de’ story!”
ðŸ™‚
N

Posted by: Nate The Surveyor
Now we’re getting the “rest o’ de’ story!”
ðŸ™‚
N
Yes, I wasn’t trying to hide anything, just not to confuse with extra information.
The basic problem still stands as in the OP… I have a file with Lat/long, which I have now converted to E/N, but there is also the project coordinates and scale factor to consider…. My converted file is OK as far as UTM goes, but not in regards to the Project Coordinates.
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