Hi everyone,
When using lat-long data and displaying it on the screen or printing it with
ps.map, in what projection is this data actually displayed on these 2D media
? Is this hardcoded into Grass ?
Moritz
M Lennert wrote:
When using lat-long data and displaying it on the screen or printing
it with ps.map, in what projection is this data actually displayed
on these 2D media ? Is this hardcoded into Grass ?
There isn't any "projection" as such. Actually, GRASS in general
doesn't really have much to do with projections; most operations work
on abstract coordinates, without regard to whether they are degrees,
metres etc.
When it comes to displaying data, GRASS maps coordinates to pixels
such that the current region just fits within the active display frame
while preserving the aspect ratio.
--
Glynn Clements <glynn.clements@virgin.net>
On 23 Mar 02, at 13:27, Glynn Clements wrote:
M Lennert wrote:
> When using lat-long data and displaying it on the screen or printing
> it with ps.map, in what projection is this data actually displayed
> on these 2D media ? Is this hardcoded into Grass ?There isn't any "projection" as such. Actually, GRASS in general
doesn't really have much to do with projections; most operations work
on abstract coordinates, without regard to whether they are degrees,
metres etc.When it comes to displaying data, GRASS maps coordinates to pixels
such that the current region just fits within the active display frame
while preserving the aspect ratio.
But it has to do so using some formula between lat-long coordinates and x-y
pixels, no ?
Moritz
On Saturday 23 March 2002 06:30, M Lennert wrote:
On 23 Mar 02, at 13:27, Glynn Clements wrote:
But it has to do so using some formula between lat-long coordinates and x-y
pixels, no ?
I think it's just an XY system, with X=decimal longitude and Y=decimal
latitude. The origin is where the equator meets the prime meridian. The
range of X is from -180 to +180 and the range of Y is from -90 to +90.
If the coordinates were scaled to a linear measure (meters) then I think it
might become a standard cylindrical projection, but I don't know which one.
Roger Miller
M Lennert wrote:
> > When using lat-long data and displaying it on the screen or printing
> > it with ps.map, in what projection is this data actually displayed
> > on these 2D media ? Is this hardcoded into Grass ?
>
> There isn't any "projection" as such. Actually, GRASS in general
> doesn't really have much to do with projections; most operations work
> on abstract coordinates, without regard to whether they are degrees,
> metres etc.
>
> When it comes to displaying data, GRASS maps coordinates to pixels
> such that the current region just fits within the active display frame
> while preserving the aspect ratio.But it has to do so using some formula between lat-long coordinates and x-y
pixels, no ?
Yes, the obvious one:
map_x = map_x0 + screen_x * scale
map_y = map_y0 + screen_y * scale
with inverse:
screen_x = (map_x - map_x0) / scale
screen_y = (map_y - map_y0) / scale
where:
+ <screen_x,screen_y> is the pixel coordinates (<0,0> is top-left),
+ <map_x,map_y> is the corresponding map coordinates (whether degrees,
metres, feet or whatever),
+ <map_x0,map_y0> is the top-left (North-West) corner of the region
and "scale" is chosen such that the region just fits into the display
frame.
If you meant to ask what kind of projection would you have to apply to
a lat-lon map to get the same "shape" on screen, the answer is Plate
Carée (aka Equidistant Cylindrical), with the latitude of true scale
being the equator.
--
Glynn Clements <glynn.clements@virgin.net>
On 23 Mar 02, at 17:30, Glynn Clements wrote:
If you meant to ask what kind of projection would you have to apply to
a lat-lon map to get the same "shape" on screen, the answer is Plate
Carée (aka Equidistant Cylindrical), with the latitude of true scale
being the equator.
Yes, that was what I was looking for. Thanks a lot Glynn !
Moritz