Merging AutoCAD drawings into GIS(having cooridinates problems)
Merging AutoCAD drawings into GIS(having cooridinates problems)
(OP)
WH (Visitor) Sep 17, 2001
I am using AutoCAD map 4.0. I am trying to export CAD files into GIS. AutoCAD map will allow you to export CAD files as shapefiles. The problem I am having is the coordinate system. The GIS database was done in TN. State coordinates. The drawings (property boundaries) were done by surveyors. The surveyors didn’t use the TN State coordinate system on the drawings. I know the drawings are not going to overlay perfect with the polygons in the GIS database. How would you get those drawings into the required state coordinates. Somehow I have to assign the property corners to a certain location in the GIS database. I have posted this message on a couple of forums. They keep saying to have the surveyors put it into the required cooridinates. That is not possible for my case. These are old drawings. We plan to scan and vectoize them. Any help would be greatly appreciated.
I am using AutoCAD map 4.0. I am trying to export CAD files into GIS. AutoCAD map will allow you to export CAD files as shapefiles. The problem I am having is the coordinate system. The GIS database was done in TN. State coordinates. The drawings (property boundaries) were done by surveyors. The surveyors didn’t use the TN State coordinate system on the drawings. I know the drawings are not going to overlay perfect with the polygons in the GIS database. How would you get those drawings into the required state coordinates. Somehow I have to assign the property corners to a certain location in the GIS database. I have posted this message on a couple of forums. They keep saying to have the surveyors put it into the required cooridinates. That is not possible for my case. These are old drawings. We plan to scan and vectoize them. Any help would be greatly appreciated.





RE: Merging AutoCAD drawings into GIS(having cooridinates problems)
Are you converting plan (ie. Y_X coordinates or is the conversion to be in 3 dimensions involving heights also?)
RE: Merging AutoCAD drawings into GIS(having cooridinates problems)
if you have min 3 or more known points on drawing(coordinates should be known) you can transform that drawing very easily. but you need those points as base points...
those points might be corners of buildngs or better ground control points(GCP) on drawing...
RE: Merging AutoCAD drawings into GIS(having cooridinates problems)
Your suggestion of three or more known common points is a very simplistic way of doing this and also VERY DANGEROUS as it depends on the accuracy of the common points you are using and other factors. I would only use this if I had no other way or if I needed it for a quick inaccurate solution.
Co-ordinate transformations from systems in GIS are often used indiscriminately thus ending in distortions on maps and then they end up blaming the survey!
The best general rule to apply is to use the BASE \ ORIGINAL data of both sources one wishes to compare or merge. Then analyse common points and obtain an idea errors or accuracy.
Regards
RE: Merging AutoCAD drawings into GIS(having cooridinates problems)
luck
RE: Merging AutoCAD drawings into GIS(having cooridinates problems)
however, it depends on accuracy, as I know these transformations are global and reliable! (affine and projective transformation). I couldnt understand that why you called very dangerous?
and if you use ground control points for transformations ,your result will be accurate. because these points were located with geodetic or photogrammetric ways and must be accurate. as you know maps are produced according to these points.
Regards
RE: Merging AutoCAD drawings into GIS(having cooridinates problems)
The other aspect is simply to "compare apples with apples" ie. yes ground control points as directly derived or obtained from survey are properly identified on the photo/map, and then used in the transformation.
I just wished to draw your attention to distortions which can occur due to aforementioned methods. Please let me know if you were succesfull in the application you are using this method for? Of course you are welcome to ask anything more you wish to know.
Good luck