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Using GPS Coordinates with a non Georeferenced drawing


Mr.Alex

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Hi,

 

I'm sorry if this has been covered before.

 

I occasionaly use a program which plugs into AutoCAD and plots points using GPS coordinates. So far it has allways worked fine, because the drawings I have been working on are 'georeferenced'.

 

One day, I know I'm going to need to plot onto a drawing which is not georeferenced, and the coordinates will be meaningless. The software has X and Y offsets to accomodate this.

 

My question is, how do you find out what the offset would be?

 

So far, I have come up with two possible solutions:

 

1. Get GPS coordinates of known points on the drawing (such as the corner of a building). This could be a pain if the site visit has allready been completed.

 

or

 

2. Get hold of a drawing which is georeferenced and covers part of the same area, and align the two. This would probably incour further costs.

 

Are there any other ways around this?

 

Many thanks,

 

Alex

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Is the target drawing not geo-referneced at all or is it referenced to a different coordinate system/datum/projection/units...?

 

If it's not referenced at all, then I think you only have the 2 options you have suggested...

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Thanks for your reply chulse.

 

It's only a theoretical problem at the moment, just something that I expect to come across at some point in the future.

 

But yes, the potential problem I am trying to solve would be one where it wasn't geo-referenced at all. As in someone just started drawing a plan randomly in the model space.

 

What about using something like Google Earth to get coordinates of a known point, such as the corner of a building? Is it possible to get coordinates that AutoCAD can use from it? maybe this could be my third option?

 

Thanks

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I have had this happen. Sometimes surveys are done in "assumed coordinates" where a bench mark is adopted and given a random coordinate (10000,10000 maybe) and everything is based on that.

 

In that case I took GPS coordinates of some known points (sidewalk corners and manholes in my case) and lined them up that way. Just know that this is never as accurate as a true survey.

 

Depending on the accuracy you need and what coordinate systems you are working in, GE might work. Also not a very accurate method.

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Yes, Google Earth would be a pretty rough fix, but I'm trying to think of a 'budget option' to be able to give to my clients if they are happy with the lack of accuracy and don't want to pay for either of the other options.

 

Thanks Chulse.

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  • 6 years later...

Hi,

 

I'm actually just wondering what the name of the plugin is that you use to plot gps points in autocad? I'm an environmental designer currently working on a wayfinding project for an airport and we just finished a sign inventory survey which included GIS information for all exterior signs. We need to pull GIS/GPS information for all interior signs now and the equipment that was being used to gather the GIS information doesn't work in interior environments. We did manage to gather three control points with gis info outside of the buildings so that we could eventually use them to for triangulation with the hopes of being able to then point to any sign location on the autocad plan and gather the necessary GIS/GPS info for that location. Im hoping that the plugin you have mentioned will meet our needs but I'm open to any other ideas as well? Thanks in advance for any advice you can offer.

 

Jeff Huffman

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Hi Jeff,

 

 

The plug in I use is for plotting trees, and is called ArborCAD. It's not particularly for GPS but can allow you to plot trees to GPS coordinates.

 

 

It might be worth looking at something like ExpertGPS? I don't know much about it but it was recommended to me a while back.

 

 

If it's just 3 points, you could do it manually? Just plot a small circle with the coordinates for the centre point to match the coordinates you have? The coordinates would need to be in the right format. But I have done this successfully in the past.

 

 

Alex

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Don't see what the problem is, here is a point straight of my drawing X = 268476.987 Y = 5774127.220 its metric this comes out of the field survey we just work in big numbers. We have old arbitrary surveys and we just try to get at least 3 points with current gps control, then just use a move and rotate or align plus a height shift to realign to gps co-ords.

 

If you want to work all nice and square off the corner of the building use UCS and set the origin to 0,0. At any time you need real world just UCS W. We do it everyday 8 staff.

 

Re google maybe 0.5m accuracy , the closer in the better, but we know from our own aerial imaging there can be errors, our GIS we would pick an allotment boundary from another layer this has been extensively rubber mapped controlled by the state not Mr Google and is continuously upgraded for accuracy.

 

Our surveyor checked his house boundary compared to our mapping and got around 150mm error with a survey quality GPS, his fishing GPS suggested around 250mm but he left it for 1/2 hour.

 

An example our survey in Metres, architects plan in mm 1st scale 1/1000 then move to known point then rotate to correct bearing. Use say 0.000000

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