v.in.arc, v.support, & v.to.rast

[rgp at abba]

> From grass-lists-owner%max.cecer.army.mil at munnari.oz.au Wed Sep 14 10:02:43 1994
> Date: Tue, 13 Sep 94 15:11:37 CDT
> From: jschuler at ncg.scs.ag.gov (Jill Schuler)
> Sender: grass-lists-owner at max.cecer.army.mil
> Reply-To: grassu-list at max.cecer.army.mil
> To: grassu-list at max.cecer.army.mil
> Subject: Re: label errors v.in.arc
> Content-Length: 2000
> 
> 
> %%%   I've managed to convert a number of coverages from arc7 to grass.  It's
> %%%work great except that a large number of polygons on each cover don't
> %%%get labeled.  It get lots of point to area errors.  Has anyone run into
> %%%this problem and been able to fix it?  I dread the thought of having to
> %%%fix this by hand... it's several thousand polygons over several coverages.
> %%%
> 
> Kurt,  
> In the past I have seen this when importing dlg's.  I will assume that the
> problem is simular.
> 
> The problem maybe that the map threshold is not set correctly in the import 
> routine in GRASS.  The import routine uses the scale to determine the 
> digitizing threshold and the snapping threshold. 
> 
> WORK AROUND *************************************
> If you go into v.digit and reset the map scale (I know this really is not 
> what you want to do) to a smaller value, then the map threshold will also 
> be smaller.  
> 
> Write out the session and exit v.digit.  Then run the file through v.support.  
> You should see a difference in the number of label errors. 
> Keep adjusting the map scale until you have resolved all the label
> errors.
> 
> In 4.0 GRASS you could change your digitizing threshold and snap threshold
> then exit v.digit.  For some reason?? 4.1 GRASS will not let you do this.
> When you exit, even after writing out a session, and re-enter v.digit the 
> default values are back.
> 
> Cheers,
> Jill
============================================================================
KURT,
     Here is a message I sent to Olga at CERL which will alert you to a 
     potential problem;a difference in how ArcInfo and Grass interpret
     topology
-----------------------------------------------------------------------------
     I have tried relabelling but to no avail.I have put the labels 
     v.support claims not to have matched to areas in a sites file
     to see where they lie.
     They look to be inside the areas which have open area lines and outside
     any islands.
     However I have noticed that v.support does not build the topology
     correctly if a redundant/bridge line is present linking an island to a
     surrounding area.
    ie. ----------
       |          |
       |          |
       |          |
       |   ---    |
       |  |   |   |
 ----->|--| x1|   |
       |  |   |   |
       |   ---    |
       |     x2   |
       |          |
        ----------
       In this case the polygon with label x2 is left with open area
       lines.ArcInfo must be able to tolerate this but the GRASS logic
       can not.Is this a difficult problem to solve or should I try
       to find these little redundant vectors ?
--------------------------------------------------------------------------------
You may also see this problem if you bring data from MicroStation.
Only solution is to remove redundant/bridge lines.
Perhaps CERL might add an option in v.support to handle this ???
Compatibility with ArcInfo would be invaluable in this situation
saving many hours of searching and editing.
If this is turns out to be your problem then EMAIL me
for an optimum editing strategy using xdigit or v.digit

Hope this will be of some help

        Rod Paterson
        Aberfoyle Resources Ltd
        Phone: 61-3-8822226
        Fax:   61-3-8131086
        Email: rgp at aberfoyle.oz.au