I have a survey (INHP-EF), and I have a georeferenced JPG file (GCS EPSG:3857). They aren't QUITE on top of each other. The image is "north" a couple of feet.
Is this possibly some sort of unit mix-up?
Or is it more likely that whatever datum the surveyors used is slightly off from whatever is governing the image?
What else can I do to check that everything is coming in exactly where it is supposed to?
My engineer is asking me why they don't match up exactly and I might be losing hair.
Hi Mary, can you try a little experiment?
go to your working drawing ref attach your survey ( you already have units and GCS set) so set your view to the extent you want covered.
now go to the google pro earth synchronise and is the image good enough for your needs in google earth?
can you kmz to see it is where you think it should be? Did your survey match exactly on the image... ? Ok then you confirmed that your original image is off and not the survey..
You could go to your holding file that has the jpg, attach the survey and correct the image through the warp tools using as many points as possible around the outer edges or as many observable points as possible then when your satisfied , in the raster manager select the raster and under the file save as GEOTIFF..
Now you can use the new GEOTIFF as your adjusted raster, without the sister file from the jpg...
Alternatively you could create empty 3d drg and exref your survey fit view to your needs, turn off survey , now goggle earth syncro and use the google earth capture tool this will create the mesh you can now trace create a polygon around the mesh extents to ref attach downloaded capture of google earth later, go to the google view and use the download image / save image tools... if you haven't changed view and zooms the down load will be a perfect match of the mesh polygon, you can delete mesh but leave the boundary you drew..
Ref attach the 3d drg with polygon to your drg that has the old raster( switch the old raster off for now detach later) , now through the ref manager ref attach interactive to match the polygon the new downloaded raster from google earth pro ( its in colour now), now export to geotiff and you now have a new raster to replace your old one and its the right coordinates and will land in the right location without the need of a sister file ( JGW) ...this should now be perfectly aligned to the survey..
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Answer Verified By: MaryB
After some more experimentation, the survey doesn't line up with the image, or any image.
In addition to the GeoJpg I was provided, I have now also (as Lorys suggested) checked against Google Earth, both by bringing in a Google Earth image and by exporting my DGN to KML/KMZ; I have also checked against Bing Maps. I see the same "offset" with everything. I also have the survey in AutoCAD, and it relates to any geo-referenced imagery the same way.
I don't think it's that big a deal - at the coordinates we're discussing, I wouldn't be surprised if there's just some rounding difference between the image sources and the surveyor's datum.
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That size error is the stuff of nightmares. Nice pick-up, good idea checking against a third source. Keep in mind Google can be off a lot, but you've obviously done what you needed to verify. One other snag is the international vs US Survey Foot.
Geo referencing is just wonderful, but it can be really tough to spot two feet of error.
boy am I Glad only have to deal with just meters and not survey ft and feet internat...