Telstra is the largest telco in Australia and was once fully owned by the
federal government as part of the post office. They used GRASS for
analysis.. as below GRASS only in research institutions.. Not in this case.
Apparently they double-checked the results with published values, and a
distance given in decimal degrees is obviously wrong, so I don't think they
made any mistakes. My guess is that they actually reprojected the lat/lons for
the processing or created a v.dist output=line_segments map and used something
like d.what.vect (which does use geodetic distance) to extract the distances.
Regardless, it would be nice to fix 'v.distance upload=dist,to_along' to use
geodetic distance; check the nearest feature calculation isn't biasing to
features in the N-S plane at high latitudes; and more generally make the vector
modules & libraries more Lat/Lon aware.
Or start adding checks into the code of any module which could output a
misleading result:
if (G_projection () == PROJECTION_LL)
G_fatal_error(
_("%s works in Euclidean space. Please project your data."),
G_program_name() );
Hamish
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Regardless, it would be nice to fix 'v.distance upload=dist,to_along'
Some time ago I asked on GRASS user ml about to_along and
to_angle in v.distance (see below), because I don't
understand how they work, and documenation is lacking here.
You seem to know more. If you can shed some light on this
please do, and I'll update docs.
Maciek wrote:
"to_angle: angle of linear feature in nearest point,
counterclockwise from positive x axis, in radians, which is
between -PI and PI inclusive"
Anybody knows what is "angle of linear feature in nearest
point" supposed to mean?
BTW - another thing I don't understand in v.distance is the
to_along option: "to_along: distance to the nearest point on
'from' feature along linear feature". Anybody knows if the
"linear feature" is supposed to be present in vector "from"
or vector "to"?
'to_along' gives distance measured along linear feature (from it's
begining) to the point on that feature (blue point) given by
point/centroid from 'to' map (highlighted part of line in the attached
picture). 'to_angle' is angle of line in that point (blue point on
line) (radians, counter clockwise from x axis)
Martin
2007/10/26, Maciej Sieczka <tutey@o2.pl>:
Hamish,
> Regardless, it would be nice to fix 'v.distance upload=dist,to_along'
Some time ago I asked on GRASS user ml about to_along and
to_angle in v.distance (see below), because I don't
understand how they work, and documenation is lacking here.
You seem to know more. If you can shed some light on this
please do, and I'll update docs.
Maciek wrote:
>> "to_angle: angle of linear feature in nearest point,
>> counterclockwise from positive x axis, in radians, which is
>> between -PI and PI inclusive"
>>
>> Anybody knows what is "angle of linear feature in nearest
>> point" supposed to mean?
>>
>> BTW - another thing I don't understand in v.distance is the
>> to_along option: "to_along: distance to the nearest point on
>> 'from' feature along linear feature". Anybody knows if the
>> "linear feature" is supposed to be present in vector "from"
>> or vector "to"?
