[GRASSLIST:2532] Re: Which projection?

[Gordon Keith]

On Sat, 29 Sep 2001 00:08, Rich Shepard wrote:
> On Fri, 28 Sep 2001, Gordon Keith wrote:
> > I have a gridded data set I want to read into grass, but I'm not
> > sure which projection to use. The header for the data contains the
> > following information:
> >
> > Coordinate system: Lat/Long
> >
> > I can get the same data output using a different coordinate system
> > with the header:
> >
> > Coordinate system: merc


>   The second set of values appears to be using the Mercator
> projection with a coordinate system based on metres (or meters, let's
> not argue). 

Both sets of data are using the same projection with different 
coordinate systems (one degrees, one metres).

> Regardless, GRASS should read your lat/lon values
> quite well. Are you using a sites (s.*) module? Do your data
> represent elevations? If so, look at one of the DEM-importing
> modules.

The data is elevations (in this case). I have been using s.in.ascii and 
s.surf.rast to input the raw data.

I'm trying to import the output of another package which has gridded 
the data, so that I can directly compare any work I do in GRASS with 
the products from the other package. So in this case the data itself is 
a raster, but output as x y z triplets. 

> > The data is gridded, but the grid coordinates don't follow lines of
> > latitude and longitude, so I haven't had good results reading the
> > data into grass using a latitude longitude projection.
>
>   And they wouldn't if they were collected by satellite, aerial
> photography, or someone walking/driving across the landscape. Just
> because they're on a grid does not require the grid to be precisely
> laid out along lines of latitude or longitude.

The data is bathymetry (sea bottom depth) data collected from a multi 
beam echosounder (swath mapper). The data I'm trying to import is the 
median depth for each 30m square. The data has been gridded into 30m 
squares precisely laid out along lines. But the lines don't follow 
lines of latitude or longitude. I'm assuming that to get 30m squares 
they have projected the data. 

>head -30 2_Median.xyz.ascii
*** Neptune Ascii file from Kongsberg Simrad A/S ***
Survey name: Broken35v
Processing operator name: gjk
Datum: WGS84
Half axis: 6378137.0000000
Flattening: 1/298.25722356300
Coordinate system: Lat/Long
Latitude min.: -38.03029440
Longitude min.: 149.23125830
Latitude max.: -37.92062415
Longitude max.: 149.34317380
Latitude cell size: 30.00 meter
Longitude cell size: 30.00 meter
Median depth of each cell
 
149.2493649 -37.9815254 116.31
149.2494295 -37.9815420 116.04
149.2499221 -37.9815523 115.98
149.2502370 -37.9816029 116.01
149.2505497 -37.9815990 115.94
149.2508048 -37.9816694 115.83
149.2513027 -37.9815896 115.76
149.2514899 -37.9815561 115.80
149.2518612 -37.9814841 115.83
149.2522434 -37.9815467 115.86
149.2525526 -37.9816121 115.84
149.2527993 -37.9816475 115.96
149.2533594 -37.9815329 116.17
149.2492347 -37.9813986 116.26
149.2494824 -37.9812065 116.01

What I'm trying to do is create a grass raster where each cell of the 
raster matches one cell of the output. But I haven't been able to work 
out what region to use in grass to match the output. (Note that the 
region contains a lot of null cells which don't have any entries in the 
output)

I was thinking that if I could set up the right region in grass a 
simple s.in.ascii, s.to.rast would get me there. 

Any other suggestions?

Regards
Gordon