[Qgis-user] how to size a vector layer polygon
flaplante at flos-inc.com
Wed Oct 21 12:53:23 EDT 2009
Hi Craig, thanks for the reply.
First item to be aware of is that this a rural area, about 30% farm
land, the rest forested. There is NO official surveyed benchmark in the
area, only one state bridge, and one highway intersection.
The scanned maps in question are property tax maps (8-1/2 x 11 reduced
in 2 stages from original size D). I used the GeoReference plugin for
qGIS. The maps in most cases have been rotated by the author to fit the
paper, rather than having North at the top. Also most of the 15 maps
have few recognizable features that can be recognized on either a
Topographic map or an aerial photo - some as few as three. I have used
the highest degree polynomial possible in every case, but they are
seldom evenly spread over each surface. The resulting distortion is
terrible; I find that many of the resulting land parcel boundaries are
in the middle of obviously plowed fields, some run through the middle of
I have redrawn all the roads using aerial photos as a guide, adding
proper right of way buffers and am now in the process of adjusting
boundaries to fit the roads. In some cases, but not all, there are
dimensions for the parcel sides. It is those I am trying to get to the
proper size. My hope is that by getting those sides to the proper
dimensions, and adjusting others to hedgerows, stream center lines
(where appropriate), etc. I can get a reasonably good approximation of
the land parcels.
I got a lead on the program "OpenJump" which indeed will show line
lengths (and direction) when you draw a line or polygon. Unfortunately
it seems not to have any means to work with mixed projections. If I can
get it to work I will probably use OpenJump to adjust my shapefiles, and
then pull into qGIS. But I would prefer to stay in a single software
package if possible. I had hoped someone would point me to a fix or
update to the Azimuth & distance plugin.
Craig Leat wrote:
> Fred LaPlante wrote:
>> I am trying to adjust a set of property maps based on georeferenced
>> scans of paper maps. While the georeferenced images aren't bad as
>> pictures, they are poor regarding dimensional accuracy.
> What method did you use to georeference your scanned maps? If you
> haven't tried it already, try a higher order polynomial. I think you
> will want to georeference the scanned map as best you can rather than
> trying to manipulate the vector map. In case of the latter you would
> be better off in a CAD app.
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