[GRASS5] Bug: vector maps shifted (d.area / d.vect)

Glynn Clements glynn.clements at virgin.net
Sat Oct 20 07:44:41 EDT 2001

Markus Neteler wrote:

> just found an inconsistency: d.area draws maps
> shifted to north-west.
> I have generated a polygon with r.digit, the vectorized
> with r.poly. Comparing d.vect and d.rast the overlay
> looks perfect, while d.area produces a shifted result.
> Also v.area is not consistent (I guess d.area and v.area
> have a similar code history).

d.vect uses G_plot_line() (in src/libes/gis/plot.c), which (in
fastline()) adds 0.5 to each coordinate. d.area doesn't use this

If you remove the +0.5 from fastline(), d.vect and d.area coincide. 
However, G_plot_line() is used in many places, so changing it to suit
d.vect may break other things.

Glynn Clements <glynn.clements at virgin.net>

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