topic Re: Expected Accuracy of Geolocation Imagery in Civil 3D Forum

Expected Accuracy of Geolocation Imagery

idigg-be — Tue, 09 Apr 2024 17:29:59 GMT

we have a surveyor giving us points that he claims are on state plane. when bringing them in, they are anywhere from 10-15' away from the geolocation imagery.

I have brought in gis data from our state's official source, including LIDAR contours, imagry, and breaklines. that all agrees with the geolocation imagery built into autocad.

what degree of accuracy can i expect. The surveyor is insisting that it is on state plane. from what i understand from numerous other contacts in the field.. if the points were on state plane, they would be spot on. maybe a foot or 2.

at what degree of inaccuracy can i confidently state that no, these points are not on state plane?

is there anything on my end that i might be doing to not have the correct location? drawing is set to the correct zone(nad83 pa south zone, foot), projection lm, datum nad83. i have not used the "mark position" or "edit location" or any other options in the geolocation tab.

Re: Expected Accuracy of Geolocation Imagery

cneelyUFTR2 — Tue, 09 Apr 2024 17:29:20 GMT

Personally I never trust aerials to be that precise. It might be a difference between Surface-Grid, but it's usually a greater difference than 10'-15'. What is the geographic area you are in? Post the drawing and GIS data if you can.

Re: Expected Accuracy of Geolocation Imagery

idigg-be — Tue, 09 Apr 2024 17:32:25 GMT

im not looking for anything that "precise", but when i get drawings from other reputable companies, their files all overlay almost exactly on the other data we insert. it seems to be only info from this guy that we have issues with. btw, i was editing my original post with more info when you responded. please check it out.

Re: Expected Accuracy of Geolocation Imagery

cneelyUFTR2 — Tue, 09 Apr 2024 17:34:19 GMT

Both can be in the same State Plane but if one is surface (ground) and the other is grid (plane) they wont line up. Need more info to be able to tell however.

Re: Expected Accuracy of Geolocation Imagery

_Hathaway — Tue, 09 Apr 2024 17:41:45 GMT

It 'should' be on state plan if they claim so. 10-15' wouldn't normally be a grid to ground conversation and if it was there wouldn't be a valid reason to bring it to ground. The geolocation tool is somewhat accurate but by no means should be the ultimate decider of state plane or not. I would provide them with the information you have uncovered and let the producer of the data respond.

Re: Expected Accuracy of Geolocation Imagery

Pointdump — Tue, 09 Apr 2024 18:04:15 GMT

Hi Wes,
Might also be a difference of datums: NAD83, NAD83(HARN), NAD83(NSRS2011), etc...
Dave

Re: Expected Accuracy of Geolocation Imagery

idigg-be — Tue, 09 Apr 2024 18:30:45 GMT

@Pointdump wrote:
Hi Wes,
Might also be a difference of datums: NAD83, NAD83(HARN), NAD83(NSRS2011), etc...
Dave

as far as i know people only use nad83 in our area. ive never heard of harn, 2011, etc. So i would assume WE are on the correct datum... but idk about him. is that something i should ask him?

not sure if it is relevant, but it is not just a horizontal shift, but it is also on a different rotation.

Re: Expected Accuracy of Geolocation Imagery

idigg-be — Tue, 09 Apr 2024 18:12:32 GMT

@cneelyUFTR2 wrote:
Both can be in the same State Plane but if one is surface (ground) and the other is grid (plane) they wont line up. Need more info to be able to tell however.

im not sure what the grid/ground reference is, but it is not just a shift; it is also a rotation issue. i will try to get the files available when i get a break at work. should i just insert the points over some lidar info and share that dwg?

Re: Expected Accuracy of Geolocation Imagery

idigg-be — Tue, 09 Apr 2024 18:45:20 GMT

@_Hathaway wrote:
It 'should' be on state plan if they claim so. 10-15' wouldn't normally be a grid to ground conversation and if it was there wouldn't be a valid reason to bring it to ground. The geolocation tool is somewhat accurate but by no means should be the ultimate decider of state plane or not. I would provide them with the information you have uncovered and let the producer of the data respond.

so grid vs ground is above my knowledge base. im not sure how or why one would bring it to ground.

also, as mentioned elsewhere, there was also a rotation issue.

i have literally showed the surveyor what the issue is and he has insisted that it is state plane. he is a contractor that we use to do our surveying. not a company, not an employee. just a guy that we hire to do the work.

the burden of proof is on me to convince everyone around me that something is not correct.

dwg is attatched. using the utility poles as reference you can see that the points are not just shifted. and my mistake... it is actually a ~50' horizontal error.

Re: Expected Accuracy of Geolocation Imagery

idigg-be — Tue, 09 Apr 2024 18:48:26 GMT

@cneelyUFTR2 @Pointdump i posted the dwg above. just a portion of their points, and the white lines are our states gis breakline data inserted with mapimport..

the horizontal error is actually 50'. and as you can see, not on the same rotation.

Re: Expected Accuracy of Geolocation Imagery

cneelyUFTR2 — Tue, 09 Apr 2024 18:51:46 GMT

Is that the actual drawing you received from the surveyor or one you created from that data?

Re: Expected Accuracy of Geolocation Imagery

tcorey — Tue, 09 Apr 2024 18:54:41 GMT

That size bust in our geo is almost always caused by having one data set in International Feet and one set in US Survey Feet. It's a minuscule percentage difference but it adds up because of being millions of feet from the origin.

Re: Expected Accuracy of Geolocation Imagery

idigg-be — Tue, 09 Apr 2024 18:55:56 GMT

@cneelyUFTR2 wrote:
Is that the actual drawing you received from the surveyor or one you created from that data?

the surveyor sends us a txt file with pnezd points in it and nothing more. i took that, inserted it into our dwg, turned on geolocation imagry and inserted breaklines from https://maps.psiee.psu.edu/ImageryNavigator/

drew 4 lines connecting his utility pole shots to the approximate location of the utility poles on the aerial...saved and uploaded.

Re: Expected Accuracy of Geolocation Imagery

cneelyUFTR2 — Tue, 09 Apr 2024 19:12:22 GMT

The line in the screen shot below is in the middle of the street.

Ask the surveyor for a Combined Scale Factor.

Re: Expected Accuracy of Geolocation Imagery

idigg-be — Tue, 09 Apr 2024 19:22:02 GMT

when i open the dwg it shows as directly over the utility pole. not sure what is causing the discrepency..

Re: Expected Accuracy of Geolocation Imagery

cneelyUFTR2 — Tue, 09 Apr 2024 20:41:39 GMT

Well that was unexpected. Try to get the Combined Scale Factor. That could likely fix things.

Re: Expected Accuracy of Geolocation Imagery

idigg-be — Tue, 09 Apr 2024 20:45:18 GMT

@cneelyUFTR2 wrote:
Well that was unexpected. Try to get the Combined Scale Factor. That could likely fix things.

i have no idea what that means or how to ask him for it.

is it a per job thing or just a file that he is using, or a calculation he is doing?

Re: Expected Accuracy of Geolocation Imagery

cneelyUFTR2 — Tue, 09 Apr 2024 21:05:17 GMT

Phrase it as "Can you provide the Combined Scale Factor for this project?". He should know what it is.

Re: Expected Accuracy of Geolocation Imagery

Jerry_Barnes — Tue, 09 Apr 2024 21:45:20 GMT

When I open the drawing, I get what idigg-be got... it looks pretty dead-on to me.

Re: Expected Accuracy of Geolocation Imagery

_Hathaway — Tue, 09 Apr 2024 23:54:15 GMT

@idigg-be wrote:
he is a contractor that we use to do our surveying. not a company, not an employee. just a guy that we hire to do the work.

the burden of proof is on me to convince everyone around me that something is not correct.

A contractor doing surveying...what could go wrong? Not to diminish his talents but contractors should contract, not survey. It's obvious there is a problem, a problem a competent surveyor should be able to figure out quickly. I know this doesn't help your cause but you should let your manager know the situation. If they say proceed, document it and proceed.