How to achieve precisely correct rotation when using the Survey Point Tool to georeference an existing drawing?

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The scenario is this:

 

- I have an existing drawing.

- The drawing grid is NOT aligned to true north.

- I have already georeferenced it to "near enough" using the Gelocate tool. That lets me set a rotation of the globe behind my drawing, using visual references.

- I now want to precisely geolocate the drawing, using points 1, 2 and 3 within the drawing that I know the exact BNG (British National Grid) co-ordinates of.

 

I have tried to do this using the Survey point tool. In the SP tool I can specify my three points - the main survey point (using my known point 1) and the two reference points A and B (using my known points 2 and 3). The diagram below indicates the result (exaggerated for clarity).

 

The main survey point matches my known point 1 and that's fine. That point in the drawing will now report the correct BNG co-ordinates. However, the rotation of the globe is not quite correct, an inevitable result of me doing the original geolocation visually based on satellite images etc. So, survey points A and B do not exactly match known points 2 and 3. Everything needs to be rotated around the main survey point, by the angle I've indicated in blue.

 

How do I do this? I can't go back to the Geolocate tool because it's disabled now I am using a Survey Point.

 

Is my only option to unlock the Survey Point settings and manually change the "angle to true north" until I get as close a match as possible?

 

 

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Well, I have come to the conclusion that this is not possible - you just have to adjust the rotation by trial and error until you get as good a fit as you can.

 

Seems odd to me as it ought to be possible to provide two or more known points, indicate what points they should match on the drawing, and compute a best fit.

[Tom W.]

Just trying to get my head around this... So is the existing drawing aligned the way you want it (angle to true north) from the outset? And it's a case of rotating the globe to match (via angle to true north georeference setting)? Isn't it then just a case of adding stakes with real world coordinates taken from the drawing + comparing the difference in angle between the two (i.e. between the real world coordinates + their representation in the drawing), then applying this angle as the angle to true north (to rotate the globe to match the drawing)? Then move the drawing so it lines up with the stakes.

 

The survey point I think just offers an alternative way of affecting this rotation (compared to the setting in Doc Settings) plus it includes the two reference points which allow you to check that the rotation is correct.

 

I may be misunderstanding the situation + only have a fairly tenuous grasp on how all this works anyway but keen to understand it better.

 

Can you post a file?

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1 hour ago, Tom W. said:

Just trying to get my head around this... So is the existing drawing aligned the way you want it (angle to true north) from the outset?

Yup.

 

1 hour ago, Tom W. said:

And it's a case of rotating the globe to match (via angle to true north georeference setting)?

Basically yes.

 

1 hour ago, Tom W. said:

Isn't it then just a case of adding stakes with real world coordinates taken from the drawing + comparing the difference in angle between the two (i.e. between the real world coordinates + their representation in the drawing), then applying this angle as the angle to true north (to rotate the globe to match the drawing)? Then move the drawing so it lines up with the stakes.

 

That's right in principle. However I had hoped the tool would do some of this work for me, given that it offers the two "reference" points in addition to the main survey point. Additionally there is always going to be a bit of error, so I'm looking for a best fit, and this seems like something a computer should be able to calculate for me.

 

There's another thing though, which is that as far as I can work out, neither the "Stake" tool or the "GIS Stake" tool seems to be able to report BNG co-ordinates, but the "Survey Point" tool can (not that it's very easy to figure out how - you have to know that you need to choose "Cartesian Coordinates").

 

This particular project, the survey I have is geolocated with reference to the BNG. I could go through the process of marking points with one of those stake tools, and trying to do the conversion to a different co-ordinate system myself, but that's then massively prone to me doing something wrong. I'd much rather the Survey Point tool marks the correct points using BNG co-ordinates and then I check directly against that.

 

2 hours ago, Tom W. said:

Can you post a file?

 

Not easily in this case unfortunately.

[Tom W.]

1 hour ago, line-weight said:

as far as I can work out, neither the "Stake" tool or the "GIS Stake" tool seems to be able to report BNG co-ordinates

 

Just choose easting/northing option:

 

1 hour ago, line-weight said:

That's right in principle. However I had hoped the tool would do some of this work for me, given that it offers the two "reference" points in addition to the main survey point. Additionally there is always going to be a bit of error, so I'm looking for a best fit, and this seems like something a computer should be able to calculate for me.

 

I may be missing something but the computer is never going to know how far your drawing is off true north, you will always have to tell it that. The two additional survey points just allow you to check that the angle of rotation is correct, something you can't do with only one reference point obviously.

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1 hour ago, Tom W. said:

Just choose easting/northing option:

 

Ok, I think there is some kind of bug here (at least in VW2025).

 

If I place a stake and choose that option first, then I get BNG co-ordinates.

 

But if I place a stake and first choose lat/long, and then change that to EN, it doesn't give me BNG co-ordinates it gives me a version of the lat/long values

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1 hour ago, Tom W. said:

I may be missing something but the computer is never going to know how far your drawing is off true north, you will always have to tell it that. The two additional survey points just allow you to check that the angle of rotation is correct, something you can't do with only one reference point obviously.

 

If I indicate just two points on the drawing, and tell the computer what their real world co-ordinates are, it should be able to locate the drawing and determine its rotation.

 

The survey point tool actually asks for three points in total - the main point plus the two reference points. When I use the tool, I tell the computer, the main point is just here [click] on the drawing. There could be a second step where I say ...and the first of the reference points should be here [click]. As long as those two are correct it should be able to place and rotate the "globe" in exactly the right place.

 

I think that what I find confusing about the tool is that the main point "does something" while the other two don't - they just appear on the drawing and it's then up to me to go and see if they have landed in the right place. It's also not clear why it asks for two - only one is needed to determine rotation. To me, the only reason for using more than two points in total would be if you wanted to do a best fit to multiple points, each of which will have a small amount of error in them.

[Tom W.]

2 hours ago, line-weight said:

I think that what I find confusing about the tool is that the main point "does something" while the other two don't - they just appear on the drawing and it's then up to me to go and see if they have landed in the right place. It's also not clear why it asks for two - only one is needed to determine rotation. To me, the only reason for using more than two points in total would be if you wanted to do a best fit to multiple points, each of which will have a small amount of error in them.

 

I imagine the idea of having two reference points rather than one is to ensure that the correct geographical coordinate system is being used + there's no distortion...?

 

2 hours ago, line-weight said:

If I indicate just two points on the drawing, and tell the computer what their real world co-ordinates are, it should be able to locate the drawing and determine its rotation.

 

The survey point tool actually asks for three points in total - the main point plus the two reference points. When I use the tool, I tell the computer, the main point is just here [click] on the drawing. There could be a second step where I say ...and the first of the reference points should be here [click]. As long as those two are correct it should be able to place and rotate the "globe" in exactly the right place.

 

I always model with north pointing north but I did play around with the Survey Point tool when it came out + what I did was model a building in a blank file, square to the X/Y axes. Then I subsequently imported a survey file + wanted to locate the survey geometry + the model geometry correctly geographically whilst keeping the model square to the X/Y axes. So I selected the survey geometry + rotated it (using Rotate Tool) to align it with the building so the building was sitting in the landscape the way I wanted it, taking note of the angle of rotation. Then I activated the Survey Stake tool + entered 3 known coordinates from the survey plus the angle of rotation + placed the Survey Point on the first coordinate + this had the effect of georefencing the file + setting the angle to true north retrospectively i.e. locating + rotating the globe so it was correct relative to my survey + building. That the two reference points aligned with the known coordinates on my survey geometry let me know that everything was correct. I'm not entirely sure what you're saying ought to work differently: that rather than entering all the coordinates in the Survey Point dialog then clicking in the drawing to indicate where the first coordinate should be located we should click on that point first, enter the coordinate in a dialog, click on a second point, enter the coordinate in a second dialog, then click on a third point then enter the coordinate in a third dialog? 

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19 minutes ago, Tom W. said:

that rather than entering all the coordinates in the Survey Point dialog then clicking in the drawing to indicate where the first coordinate should be located we should click on that point first, enter the coordinate in a dialog, click on a second point, enter the coordinate in a second dialog, then click on a third point then enter the coordinate in a third dialog? 

 

Yes, something like that.

 

Alternatively, I think the Help page could be a bit more informative on exactly what the "reference points" are and what they do/don't do. 

 

https://app-help.vectorworks.net/2025/eng/VW2025_Guide/Georeference/Adding_a_survey_point.htm?

 

Are they intended to help set the correct rotation? If so, then it would be good if that procedure was shown on the accompanying video for example.

 

The fact that they are described as being used for "triangulation" I think makes it seem to a new user that they are going to do more than they actually do.

 

Maybe it's just me but I find a lot of this GIS stuff fundamentally confusing, because I'm not familiar with things like CRSs and so on. There's quite a bit to get your head around, in terms of understanding what's moving relative to what (it's very useful to realise that really all these translations and rotations are happening to the "globe" in the background, not the stuff you've drawn). And the consequences of getting it wrong feel like they are liable to be fairly substantial. For all these reasons, I think it's particularly important for the documentation to be clear.

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34 minutes ago, Tom W. said:

I imagine the idea of having two reference points rather than one is to ensure that the correct geographical coordinate system is being used + there's no distortion...?

In which case, it would be great to have this properly explained somewhere in the documentation.

[Tom W.]

6 minutes ago, line-weight said:

 

Yes, something like that.

 

Alternatively, I think the Help page could be a bit more informative on exactly what the "reference points" are and what they do/don't do. 

 

https://app-help.vectorworks.net/2025/eng/VW2025_Guide/Georeference/Adding_a_survey_point.htm?

 

Are they intended to help set the correct rotation? If so, then it would be good if that procedure was shown on the accompanying video for example.

 

The fact that they are described as being used for "triangulation" I think makes it seem to a new user that they are going to do more than they actually do.

 

Maybe it's just me but I find a lot of this GIS stuff fundamentally confusing, because I'm not familiar with things like CRSs and so on. There's quite a bit to get your head around, in terms of understanding what's moving relative to what (it's very useful to realise that really all these translations and rotations are happening to the "globe" in the background, not the stuff you've drawn). And the consequences of getting it wrong feel like they are liable to be fairly substantial. For all these reasons, I think it's particularly important for the documentation to be clear.

 

Not sure if this is helpful: