[GRASS-dev] v.clean meters against feets

GRASS GIS GRASS GIS developer
1

Hi everybody,

I'm testing v.clean with meters and feet projection system.
I'm trying to run v.clean in the same map and with the same
parameters, but I obtain a different result. For the testing I'm using
zipcodes_wake map of nc_spm_08

Here my procedure:

- download nc_spf location from here [0]
- reproject zipcodes_wake from nc_spm_08 to nc_spf

- run v.clean on meters location with "v.clean in=zipcodes_wake
out=zipcodes_wake_clean tool=rmarea thre=10000000" the result is

Number of nodes: 106
Number of primitives: 166
Number of points: 0
Number of lines: 0
Number of boundaries: 135
Number of centroids: 31
Number of areas: 31
Number of isles: 2

- run v.clean on feet location with "v.clean in=zipcodes_wake
out=zipcodes_wake_clean tool=rmarea thre=107638674"

Number of nodes: 84
Number of primitives: 103
Number of points: 0
Number of lines: 0
Number of boundaries: 93
Number of centroids: 10
Number of areas: 10
Number of isles: 1

The threshold in feet should be the same of the meters one because:

1 meters is 0.3048006096012192 feet (according with g.proj output)

and

10000000 squared meters is 107638674 squared feet ( 10000000 / 0.3048^2 )

I expected the same output map, but this not happen; I'm wrong or
there is something wrong in the v.clean code?

[0] http://grass.osgeo.org/sampledata/north_carolina/nc_spf.tar.gz

--
ciao
Luca

http://gis.cri.fmach.it/delucchi/
www.lucadelu.org

2

On 27/08/14 10:01, Luca Delucchi wrote:

Hi everybody,

I'm testing v.clean with meters and feet projection system.
I'm trying to run v.clean in the same map and with the same
parameters, but I obtain a different result. For the testing I'm using
zipcodes_wake map of nc_spm_08

Here my procedure:

- download nc_spf location from here [0]
- reproject zipcodes_wake from nc_spm_08 to nc_spf

- run v.clean on meters location with "v.clean in=zipcodes_wake
out=zipcodes_wake_clean tool=rmarea thre=10000000" the result is

Number of nodes: 106
Number of primitives: 166
Number of points: 0
Number of lines: 0
Number of boundaries: 135
Number of centroids: 31
Number of areas: 31
Number of isles: 2

- run v.clean on feet location with "v.clean in=zipcodes_wake
out=zipcodes_wake_clean tool=rmarea thre=107638674"

Number of nodes: 84
Number of primitives: 103
Number of points: 0
Number of lines: 0
Number of boundaries: 93
Number of centroids: 10
Number of areas: 10
Number of isles: 1

The threshold in feet should be the same of the meters one because:

1 meters is 0.3048006096012192 feet (according with g.proj output)

and

10000000 squared meters is 107638674 squared feet ( 10000000 / 0.3048^2 )

I expected the same output map, but this not happen; I'm wrong or
there is something wrong in the v.clean code?

Have you compared the topo information of the two files before v.clean ? Maybe reprojection changes some of the topology (i.e. some lines do not close, or something similar) ?

Moritz

3

On Wed, Aug 27, 2014 at 10:01 AM, Luca Delucchi <lucadeluge@gmail.com> wrote:

Hi everybody,

I'm testing v.clean with meters and feet projection system.
I'm trying to run v.clean in the same map and with the same
parameters, but I obtain a different result. For the testing I'm using
zipcodes_wake map of nc_spm_08

Here my procedure:

- download nc_spf location from here [0]
- reproject zipcodes_wake from nc_spm_08 to nc_spf

- run v.clean on meters location with "v.clean in=zipcodes_wake
out=zipcodes_wake_clean tool=rmarea thre=10000000" the result is

Number of nodes: 106
Number of primitives: 166
Number of points: 0
Number of lines: 0
Number of boundaries: 135
Number of centroids: 31
Number of areas: 31
Number of isles: 2

- run v.clean on feet location with "v.clean in=zipcodes_wake
out=zipcodes_wake_clean tool=rmarea thre=107638674"

Number of nodes: 84
Number of primitives: 103
Number of points: 0
Number of lines: 0
Number of boundaries: 93
Number of centroids: 10
Number of areas: 10
Number of isles: 1

The threshold in feet should be the same of the meters one because:

The threshold must always be in meters, also if the location's units
are not meters, because G_area_of_polygon() is used to calculate area
sizes which always returns square meters. This was wrong in the manual
of v.clean, fixed in r62047,8 (trunk, relbr70).

Markus M

1 meters is 0.3048006096012192 feet (according with g.proj output)

and

10000000 squared meters is 107638674 squared feet ( 10000000 / 0.3048^2 )

I expected the same output map, but this not happen; I'm wrong or
there is something wrong in the v.clean code?

[0] http://grass.osgeo.org/sampledata/north_carolina/nc_spf.tar.gz

--
ciao
Luca

http://gis.cri.fmach.it/delucchi/
www.lucadelu.org
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4

On Mon, Sep 22, 2014 at 9:04 AM, Markus Metz
<markus.metz.giswork@gmail.com> wrote:

The threshold must always be in meters, also if the location's units
are not meters, because G_area_of_polygon() is used to calculate area
sizes which always returns square meters. This was wrong in the manual
of v.clean, fixed in r62047,8 (trunk, relbr70).

Thanks for the clarification. I'd suggest to then also fix the
parameter description since "map units" may be ambiguous in some
cases:

threshold=float[,float,...]
  Threshold in map units, one value for each toolDefault: 0.0[,0.0,...])

markusN

5

On 22 September 2014 09:04, Markus Metz <markus.metz.giswork@gmail.com> wrote:

The threshold must always be in meters, also if the location's units
are not meters, because G_area_of_polygon() is used to calculate area
sizes which always returns square meters. This was wrong in the manual
of v.clean, fixed in r62047,8 (trunk, relbr70).

Ok, now the results are identical
Thanks

Markus M

--
ciao
Luca

http://gis.cri.fmach.it/delucchi/
www.lucadelu.org