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Deviation between Mercartors Projection and Land survey

Deviation between Mercartors Projection and Land survey

Deviation between Mercartors Projection and Land survey

(OP)
Hello there folks !!

I am new to this forum, and am excited to be part of this forum. Recently I was given an assignment to overlap a Google earth satellite image and a cluster of stitched topo survey layout over an area of 4 Kmsq. I noticed that what I am asked to do is not correct because I believe Google earth is based to be created on Mercator's or equivalent cylindrical projection technique wherein in the topo drawing isn't. I am keen to understand the least deviation that would occur in a projected earth image over an unprojected drawing over the said area.. Or I am i wrong here in my assessment ?

Thanks in advance.

Engineering begins where Architecture ends !

RE: Deviation between Mercartors Projection and Land survey

(OP)
Thank you for your reply @IRstuff..

I agree with your points, but would it be safe to say that overlapping topo survey and google earth image is the right idea ?

Engineering begins where Architecture ends !

RE: Deviation between Mercartors Projection and Land survey

IF you are using KML, there shouldn't be any problem, although, Google Earth's geodetic accuracy gets worse the farther away from the US.

What kind of errors are you seeing?

TTFN (ta ta for now)
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RE: Deviation between Mercartors Projection and Land survey

(OP)
I see a difference of about say 60mt over an area of 1kmsq, if i compare the overlapped KML file to the physical data. Fortunately, physical dimensions match with Google earth but the topo layout is shifted by 60mts in one direction.

Engineering begins where Architecture ends !

RE: Deviation between Mercartors Projection and Land survey

Coords for nearly all 'significant' projects I'm involved in are in a Universal Transverse Mercator (UTM) format. This is used for large sites and project sites that may have dimensions of several kilometers (miles). Surveyors often provide UTM coords for building/equipment locations.

Dik

RE: Deviation between Mercartors Projection and Land survey

Please confirm that all coordinates are equally shifted by the same 60 m offset.

Is it possible that your surveyor is using a datum that's specific to your region of India? While "absolute" GPS coordinates are used for Google Earth, there may be legacy reasons, such as existing monuments that might tie your survey to a slightly different coordinate system

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Deviation between Mercartors Projection and Land survey

As mentioned earlier, Google Earth's imagery registration may be poor if you're not in the US or Europe. This site: http://www.uni-koeln.de/~al001/airphotose_files/hs... discusses how you can check the imagery registration. I've done that for Chandigarh VOR-DME http://ourairports.com/airports/VICG/pilot-info.ht... and the error is around 31 m:

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Deviation between Mercartors Projection and Land survey

(OP)
Okay, I believe its a mix of both, Surveying error and Google earth's photogrammetric inaccuracy.. btw thank you for your replies !!

Engineering begins where Architecture ends !

RE: Deviation between Mercartors Projection and Land survey

What software are you using to overlay the various data? If you are doing this all inside of Google Earth, I am not familiar with it's capabilities. However, if you are overlaying an image form Earth with images or topo stitched from another source in a CAD program, you will obviously get discrepancies between projections.

However, if you use a GIS application (ArcGIS, Autocad Map 3D, etc.), and each data source is properly georeferenced, the software should correctly adjust the reference data projections to whatever master projection you have specified (and things should line up).

RE: Deviation between Mercartors Projection and Land survey

(OP)
@Terryscan : Its done over a GIS application, the fact is as @Irstuff indicated, trouble is with the overlaying map.. It along with minor inaccuracies of Google earth made things little distorted.

Engineering begins where Architecture ends !

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