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NoroZorro

map doesn't match georeferenced dwg

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NoroZorro

Hello forum!

 

I got promoted now i'm with autocad 3d map 2020!

I've asked already asked duplicate question on vanilla autocad.

I read the reply, tried and it didn't help

 

The problem is that my map is slightly off my georeferenced dwg. I've incorporated two dwg both are in nad 87 csrs mtm8 coordinates. When i assign coordinates nad 87 csrs mtm8 through a MAPCSASSIGN command, map and dwg doesn't match exactly.(see PIC attached) When i assign world mercator coordinatesto my dwg map is completely off and i'm in center of atlantic ocean.

Merci beaucoup

question map.PNG

Edited by NoroZorro

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rkmcswain

Looks like perhaps your vector geometry is on surface coordinates and the imagery is on grid coordinates. 

A common issue.

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NoroZorro

What is The fix?

Edited by NoroZorro

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rkmcswain

First you have to find out if that is the problem.

 

Where did you vector linework come from?

Where in the world is this?

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NoroZorro

Thank you

 

2 Vector files( one is shape, the other is dwg file)  came from database of the city of Montreal( both of them i believe in nad83(csrs Mtm8) coordinates

 

 

Edited by NoroZorro

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eldon

I would say that your map, which is aerial photography, is the one not exact. The image is not fully rectified. For example, do you want to match the top of the building (the roof) or the bottom of the building, where it goes into the ground? these points should have the same coordinates, but you can see the separation.

 

I would always nudge the map to give a 'best fit' to your coordinated points.

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NoroZorro

Thank you for response

 

If I match the roof buildings with map, it's fine

When you say "nudge the map" how can I achieve it?

If i Nudge the map, will latitude and longitude of my interest will be precise on map?

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NoroZorro

Nudge is done by holding ctrl key and pressing arrows on keyboard

however i can't select the map,its an online map

Edited by NoroZorro

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eldon

To try and explain the apparent discrepancy between map and drawing, one should go and find out the process of creating the map.

 

The aerial images are taken individually from a great height. But the images are not orthographic and there is distortion increasing from the centre of the image. The map is made up from many images, which are put together and the surpluses discarded. So the distortion is not constant throughout the mapping.

 

I expect the positional accuracy at maximum distortion is a closely guarded commercial secret, but it would not surprise me if it were to be plus or minus several metres. So even if you could match one part of the mapping, then 500 meters away, the mapping would not match. There is nothing you can do except accept the imperfect world we live in.

 

When I said 'nudge', I did not realise there was a command to do this, but I was thinking a moving the map a little bit. If you cannot select the map, there is nothing I can advise you to do.

I would take a screen grab of the mapping so that you can manipulate it. Someone else may know different.

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NoroZorro

Is this a more precise way:

 

Google earth_save image_reset tilt and compass..georeference the image in qgis...then bring it to cad...I already tried..will try again..

 

 

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SLW210

I was going to say exactly what Eldon mentioned. Those are not "flat" images as the world (believe it or not) is not flat. If you take a bunch of zoomed in screen grabs from Google, you will (at least I never could) get them to match as one large map, much less get a bunch of CAD line work to lineup.

 

Not to sound rude, but, you need to study on how "accurate" (there will always be a distortion) flat maps and aerials  are made.

http://historymatters.gmu.edu/mse/maps/question2.html

 

I have an extremely difficult job explaining these difficulties with aerials to my bosses at work.

Some programs out there help with this, but, I have none to specifically recommend, maybe PlexEarth. Though if you need to be as accurate as possible, you should look into purchasing an appropriate aerial and not use Google Maps for this.

I line everything up by the base of the buildings, that would be the closest to ground level.
 

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NoroZorro
10 hours ago, SLW210 said:

I line everything up by the base of the buildings, that would be the closest to ground level.

when you say everything you mean offline map?Are you capturing image within autocad and then aligning it?

Can you share your workflow?

I've a lot of bosses too

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SLW210

Pretty much just like you show in your image, like eldon already mentioned, you just have to move the image to best align to your line work. You probably have more tools in Map 3D, sounds to me like you need some training with this aspect of CAD.

It just isn't as complicated as you want to make it, there is just no way to have dead accurate aerials like you want AFAIK. You just have to use best fit and/or take actual field measurements to verify.

 

The Earth is not flat, you just have to work with that aspect of things.

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NoroZorro

Thanks SLW210

 

I do need a training with autocad 3d map. For now, this is my question

 

when I adetrasnform captured online image for sake of aligning it and when there is a prompt to select object, command line is saying 1 was filtered out

 

I will investigate it later, but in case i dont find answer i decided to ask you guys

 

Found this post:

https://knowledge.autodesk.com/support/civil-3d/troubleshooting/caas/sfdcarticles/sfdcarticles/Geolocation-Issues-using-BING-Maps.html

 

later.....boss is calling

 

 

Solution:

To work around the problem:

  1. Start by opening MAPTRACKCS.
  2. Whenever possible, place cross hairs in the drawing near a known location; meaning something you can use as a marker point.
  3. Run the GEO command, when the map pops up, verify if location of state and county are correct or not.
  4. If not, check the Lat/Long listed at the very top of the GEO command window. Copy what is listed in MAPTRACKCS and paste into that top line.
  5. Use the search (magnifying glass) at the right of the top line to update the map location.
  6. Use the Drop Marker Here box (at the left side of the map).
  7. Continue (at the bottom).
  8. Select point (same as step #2 above).
  9. Enter north direction.

The following image shows commands from GEO and MAPTRACKCS with the corrected location:

 

I'm trying to follow the steps from the link above. Unfortunately, the type of coordinates i use gave me x.y in meters, therefore I can't run geo and paste those values there...gentlemans did I lose you? 

 

 

Edited by NoroZorro

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NoroZorro

Hello

1.Image has different coordinates sytem

2.Image is off by 3 meters

 

The part, that the earth is round I got.

How do you reduce the error of 3 meters?

Untitled.png

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eldon
11 hours ago, NoroZorro said:

......2.Image is off by 3 meters

 

How do you reduce the error of 3 meters?

 

 By moving the image.

 

When you consider that you have a drawing with lines to a known coordinate system, and you are overlaying an image, whose provenance is unknown and whose pixels only have a coordinate because someone matched a pixel or three with a known land mark at some place, you can only rely on the pictorial nature of the image.

 

With today's technology that is available to the public, one cannot hope to have a pixel perfect map image.

 

When an aerial image is taken, one knows the position of the camera lens, (after adjustment for perturbations in the GPS satellite). One cannot know the position of the image pixels, unless one is using LIDAR, and then one has a point cloud instead of an image. If you take a picture of a mountain in the distance, your camera can take its own position from GPS, but cannot take the position of the mountain.

 

Finally, just to muddy the waters, the Prime Meridian at Greenwich is where all Longitude is meant to be measured from. Using Google Earth, this point is visible on the ground, but the point at zero longitude (GPS) is 103 metres from this point. I recommend further investigation into coordinate systems and map projections and photogrammetric mapping.

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eldon

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TerryDotson

> GEO command ...  Use the Drop Marker

I wouldn't recommend that procedure to my worst enemy.  In real mapping you don't get to pick what your drawing coordinates are !!!  The system determines them and they vary from system to system (state planes, UTM, etc).

 

If you follow that AutoCAD procedure expect that:

  1. You will never be able to merge geometry that is on true coordinates, imported or exported.
  2. If you send the drawing to someone outside the Autodesk world where this lunacy appears correct, they will inform you that your drawing is in the wrong coordinates!  If that''s your client, you will look foolish.

 

Read reinforcing statements here.

 

Edited by TerryDotson

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NoroZorro

Hello guys!

 

There are two files:

 

34218 property line

31H05... dwg

 

both are in same coordinates system. I would like to see your results

31H05-010-3936_2016.dwg 34218.dxf

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NoroZorro

Capture.JPG

 

 

Sometimes it fits in nicely( I didn't nudge the image) it's an online map within autocad

 

 

Edited by NoroZorro

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