hello
is there any preferred grid size for creating topographic index map ?
( elevation map is produced from 1: 25000 scaled topographic map )
regards
--
Ahmet Temiz
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temiz wrote:
is there any preferred grid size for creating topographic index map ?
( elevation map is produced from 1: 25000 scaled topographic map )
[more of a thought experiment than actual educated advice]
PostScript points are 1/72", which equates to a thin line on a printed
page, about as small a feature as is useful in a hardcopy.
For a 1:25,000 scale, that equates to 347.2" or 8.8m.
So a raster resolution of 5m would be ok, but it depends on how big that
makes the data set. A 10000x10000 cell region is probably as big as
you'd want to go, but a 4000x4000 is much easier to work with so perhaps
10m resolution would be a better compromise.
From the "How to start with GRASS" help page:
"The default raster resolution (GRID RESOLUTION) has to be chosen
according to your needs. Generally, it is advisable to work in steps of
0.25 (0.25, 0.5, 1.75, 2.00, 12.25 etc.)."
I hope I understood and that answers your question,
Hamish
Hamish wrote:
temiz wrote:
is there any preferred grid size for creating topographic index map ?
( elevation map is produced from 1: 25000 scaled topographic map )
[more of a thought experiment than actual educated advice]PostScript points are 1/72", which equates to a thin line on a printed
page, about as small a feature as is useful in a hardcopy.
I thought Postscript point/line widths vary, depending on the rendering device resolution.
eg: 300x300, to 2400x2400 etc.... so you can get a 1/2400" line if you really want it (not that anyone with normal vision could see it 
A typical screen resolution (pixel) is around 1/72", so for on screen viewing these seem reasonable.
It's basically a compromise between scale/clarity & performance, so your dataset native resolution and extent, intended use, screen only or hard copy output required, and hardware specs all need to be considered to devise a suitable grid size for your purpose.
No simple right or wrong answer
Brent
For a 1:25,000 scale, that equates to 347.2" or 8.8m.
So a raster resolution of 5m would be ok, but it depends on how big that
makes the data set. A 10000x10000 cell region is probably as big as
you'd want to go, but a 4000x4000 is much easier to work with so perhaps
10m resolution would be a better compromise.>From the "How to start with GRASS" help page:
"The default raster resolution (GRID RESOLUTION) has to be chosen
according to your needs. Generally, it is advisable to work in steps of
0.25 (0.25, 0.5, 1.75, 2.00, 12.25 etc.)."I hope I understood and that answers your question,
Hamish
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Brent Wood wrote:
>> is there any preferred grid size for creating topographic index map ?
>>
>> ( elevation map is produced from 1: 25000 scaled topographic map )
>>
>
>
> [more of a thought experiment than actual educated advice]
>
>
> PostScript points are 1/72", which equates to a thin line on a printed
> page, about as small a feature as is useful in a hardcopy.I thought Postscript point/line widths vary, depending on the rendering
device resolution.
eg: 300x300, to 2400x2400 etc....
A "point" is a unit of measurement used in typography, equal to
roughly 1/72" (historically, the precise definition has varied;
nowadays, the most common definition is exactly 1/72").
PostScript's default coordinate system uses points, i.e. a file
containing the following sequence of commands will draw a one-inch
filled square, exactly one inch in from the left and bottom edges:
newpath
72 72 moveto
144 72 lineto
144 144 lineto
72 144 lineto
closepath
fill
showpage
You can change the coordinate system at will, but it starts out with
one unit equal to one point, and the initgraphics operator will reset
it to that scale.
The initial line width is one unit (i.e. 1/72" unless you change the
scaling). The current line width is stored in units, so if you change
the scaling the line width will automatically change by the same
factor. The physical resolution of the device doesn't have any affect
upon the default coordinate system or the default line width.
so you can get a 1/2400" line if you really want it (not that anyone
with normal vision could see it
PostScript allows a line width of 0 to mean a single pixel line at the
device's physical resolution. However, apart from being too thin to be
of practical use, laser printers often cannot actually print anything
that small (the electrostatic printing process tends to inherently
perform "thinning" of the black areas), so even someone with
super-hero vision wouldn't be able to see it 
--
Glynn Clements <glynn@gclements.plus.com>