[Qgis-user] how to size a vector layer polygon

[Fred LaPlante]

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 
houses, etc.

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. 

Fred LaPlante

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.
>
> Craig
>
>