topic Re: Geolocation Distortion in AutoCAD Forum

Geolocation Distortion

l_wuensch — Tue, 24 May 2022 20:51:23 GMT

I'm having an issue with adding parcel lines on the basemap in AutoCAD 2022. The parcel layer is a DXF file projected in EPSG 2284. I'm setting the location from a point KML file that is also in EPSG 2284. Also, I select 2284 for the projection in CAD. The lines appear correctly around the reference point, but there is significant distortion a short distance away. For example, the two screenshots below show how just 1,000ft away from the reference point, features are about 10ft off from their correct location (top is CAD, bottom is from QGIS). The distances are perfectly preserved; all lines seem to measure within 1ft of the line distance in GIS, no matter how far from the reference point. Is there another projection or are there different settings I can use to prevent or significantly minimize this kind of distortion?

Re: Geolocation Distortion

Pointdump — Tue, 24 May 2022 23:06:15 GMT

Hi Lynette,
The shadows are different on your 2 screenshots, so the CAD shot is Bing, but what is the QGIS shot?
In QGIS switch back-n-forth between Bing, Google, and ESRI imagery. You'll see the differences. I've seen Bing imagery "off" by as much as 12 Meters. So there might be nothing wrong with your linework.
Dave

Re: Geolocation Distortion

ChicagoLooper — Wed, 25 May 2022 12:06:49 GMT

Hello @l_wuensch

That’s good advice from @Pointdump.

For now forget about the image. Concentrate on the Lat/Long, or the geographic coordinates. Unlike aerial imagery which may or may not, be distorted, the Lat/Long will provide the verification you need.

Snap to the intersection circled in red (refer to image below). Determine the Lat/Long at each snap point—exact intersection.

No ‘I think it’s right’ or ‘It should be right’ type of snap point. The point you snapped to must be accurate. Using know-how you can get the coordinates with at least 6 decimal places. Then compare the Lat/Longs.

Re: Geolocation Distortion

l_wuensch — Wed, 25 May 2022 14:03:38 GMT

Thanks Dave. That did not even occur to me. I was using Google satellite originally in GIS. I added Bing aerials in GIS and they are a little different, see the pics below (top is Bing and bottom is Google). However, the difference in GIS is about 2ft, whereas the lines are off by about 8fft in that spot in the CAD file.

Re: Geolocation Distortion

l_wuensch — Wed, 25 May 2022 14:16:27 GMT

Thanks, ChicagoLooper. I checked the lat long in both files, two ways:

QGIS point in 2284 projection: 11288810.454288648, 3644975.6064513274

AutoCAD using "Specific": 11289000, 3644975.6065

QGIS point in WGS84: 37.33159204637364, -79.16762204263078

AutoCAD using "Geographic": 37.3316, -79.1676

The CAD coordinates aren't as precise but they seem to be pretty similar.

Re: Geolocation Distortion

Pointdump — Wed, 25 May 2022 14:44:57 GMT

Lynette,
A dxf file does not differentiate between International Feet and US Survey Feet. Something to check.
"The lines appear correctly around the reference point, but there is significant distortion a short distance away."
More details please. Reference point? Are you using command GEOLOCATION to specify that point?
Do you have anything rock solid to tie the linework to? Something like published survey control points?
If you trust your linework, I'd >>>Move Bing<<<.
Dave

Re: Geolocation Distortion

l_wuensch — Wed, 25 May 2022 16:25:42 GMT

Dave, I'm using the same Edit Location button in the ribbon as shown in the video you referenced. I used the From File option, not From Map. The file is a kml file with one point, and it is in the same state plane projection as the linework, and the CAD file. That kml is what I mean by "reference point". It works just like in the video, but my linework covers a greater area. Just a mile away from that reference point, features are about 10ft off from their location in GIS, which seems implausibly large. Also, shouldn't distortion occur with distance from the datum of the projection and not this arbitrary reference point that I am using?

Re: Geolocation Distortion

ChicagoLooper — Wed, 25 May 2022 17:17:55 GMT

Hi @l_wuensch

Why specific? It might be the same but I'd still use GEOGRAPHIC instead of 'specific.'

Furthermore, your lat/longs only have four decimals. If it were me, I'd adjust to get the maximum. To get better, more accurate Lat/Longs, you should increase your precision using the UNITS command.

Also, double check the assigned coordinate system in AutoCAD and QGIS are exactly the same. Different coordinate systems might give you different x/y values.

Re: Geolocation Distortion

ChicagoLooper — Wed, 25 May 2022 17:41:29 GMT

@l_wuensch

<<....QGIS point in 2284 projection: 11288810.454288648, 3644975.6064513274

AutoCAD using "Specific": 11289000, 3644975.6065....>>

Hmmmmm......based on what you've uploaded, the x,y coordinates don't look right.

You're off 189.5 feet in the x-direction and a fraction of an inch in the y-direction. Neither of your aerial photos are consistent with the difference those coordinate values.

Re: Geolocation Distortion

l_wuensch — Wed, 25 May 2022 19:10:52 GMT

I think that could be a result of the precision, no? For whatever reason, the latitude is given in scientific notation, see the screenshot below. Is there another way to view it?

Re: Geolocation Distortion

Pointdump — Thu, 26 May 2022 06:19:56 GMT

Lynette,
Instead of using Edit Location, have you tried just assigning the coordinate system in Drawing Settings or using MAPCSASSIGN? You can use ZOOM >> Center to find your Reference Point.
Dave

Re: Geolocation Distortion

l_wuensch — Thu, 26 May 2022 15:33:32 GMT

Hey Dave and ChicagoLooper, thanks for your helpful recommendations. I should have been more clear that I was using AutoCAD and not AutoCAD Map 3D. The lines appear in the right locations in when opened in Map 3D. It seems the spatial mismatch was due to how AutoCAD was projecting the lines on the fly.