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georeferencing dxf?

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#1
Formako

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Hello all.

I've been lurking around this forum for a while and I have gotten answers and ideas to many questions I've had in my mind, but now I'm stuck.

I have have a set dxf's that I extracted from a pdf. I know the the projection and coordinate system of these files as the pdf had a rastermap on the background and I know it's projection. However, when I set this as the projection for ArcMap and add the dxf's they are totally off and in wrong scale.

What do I need to do to get these files georeferenced correctly? I have ArcGIS 9.3, MapInfo 9.5 and Manifold 8 to work with.

Cheers!

Edited by Formako, 02 June 2009 - 01:57 AM.


#2
James Hines

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In Manifold we have something called control points. If the original PDF's have a grid & coordinates to reference then what you should do is create a blank drawing in the same projection as what your DXF's are in & place control points on the blank file in the exact coordinates where the lat/lon hairs are supposed to intersect. Of course I realize that a grid would be a dxf on it's own so the best solution is to temporary add those lines to each dxf for the purpose of geregistration.

However if the PDF does not have any grid or lat/lon coordinates you need to find rasters/ & or data to georegister your data to. The procedure is simple, look in the help file it's quite detailed, & more evolved then ArcGIS.

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#3
Nick H

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I have have a set dxf's that I extracted from a pdf. I know the the projection and coordinate system of these files as the pdf had a rastermap on the background and I know it's projection. However, when I set this as the projection for ArcMap and add the dxf's they are totally off and in wrong scale.

What do I need to do to get these files georeferenced correctly? I have ArcGIS 9.3, MapInfo 9.5 and Manifold 8 to work with.


Just a thought, and I have no idea if this will work. Convert the DXFs to shapefiles and then use FWTools' ogr2ogr to set the extents.

ogr2ogr -spat xmin ymin xmax ymax newfilename.shp originalfilename.shp

The output shapefile is not allowed to have the same name as the input shapefile.

Regards, N.
Caversham, Reading, England.

#4
Crischan

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Nick, the -spat argument for ogr2ogr is a spatial filter - coordinates are not moved by it.
Formako: What is the coordinate extent of the DXF? Is it local where one corner of the bounding box should be (0,0) or already some "real world" coordinates?
If they are local then you must georeference them before applying the CRS. ArcGIS has a Georeferencing Toolbar for that purpose.
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#5
Nick H

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Nick, the -spat argument for ogr2ogr is a spatial filter - coordinates are not moved by it.
Formako: What is the coordinate extent of the DXF? Is it local where one corner of the bounding box should be (0,0) or already some "real world" coordinates?
If they are local then you must georeference them before applying the CRS. ArcGIS has a Georeferencing Toolbar for that purpose.


Thanks Crischan, I'd just discovered this and had come back to edit my post (to say that this method doesn't work). Why on earth did I ever think it would? :( .

Regards, N.
Caversham, Reading, England.

#6
Formako

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Crischan,

It's in some local coordextent. Lower left corner is the origo (0,0).

I tried georeferencing the dxf in Manifold and it worked ok. However, the accuracy is only as good as my reference and the number of control points I input.

Would ArcGIS's Georeferencing Toolbar be better for georeferencing? I'm not very good with it.

I have also exported the dxf's to shp-format if it helps.

EDIT: one of the dxf-layers is a point layer. I can retreive the actual x- and y-coordinate for any given dxf-point in Transverse Mercator projection in wich my reference image is. Is there a way to georeference this point layer "automatically" having this information in ArcGIS or in Manifold?


Nick, the -spat argument for ogr2ogr is a spatial filter - coordinates are not moved by it.
Formako: What is the coordinate extent of the DXF? Is it local where one corner of the bounding box should be (0,0) or already some "real world" coordinates?
If they are local then you must georeference them before applying the CRS. ArcGIS has a Georeferencing Toolbar for that purpose.


Edited by Formako, 02 June 2009 - 10:58 AM.


#7
PhiNgo

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Hi Formako,

I've georeference dxfs to common coordinate systems using ArcMap 9.3. I use the georefencing toolbar to line things up. You need to know two coordinate points. If you have a boundary that's already aligned to your local coordinate system you can start editor and use the snapping function. And you don't need to convert the dxf to a shapefile. But with a shapefile you can use the spatial adjustment tool. If you don't have a boundary, select a control point and then right click and you have the option to input X-Y coordinates. Then repeat. The georefencing toolbar only allows for two control points.

Hope this makes sense.




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