[GRASS-user] Correctly Converting LL to LCC

[Glynn Clements]

Rich Shepard wrote:

>    I think that this time I have it right.
> 
>    Created a new location with the projection information based on a .prj
> file (for the hydrologic basins) and imported that map. When I look at
> PROJ_UNITS I see they are in feet:
> 
> unit: Foot
> units: Feet
> meters: 0.3048
> 
>    This leaves me with a question about the PROJ_INFO contents:
> 
> name: Lambert Conformal Conic
> proj: lcc
> datum: nad83
> ellps: grs80
> lat_1: 43
> lat_2: 45.5
> lat_0: 41.75
> lon_0: -120.5
> x_0: 399999.9999999999
> y_0: 0
> no_defs: defined
> nadgrids: WO
> 
>    The x_0 value seems to be in meters (in earlier locations it was rounded
> up to 400000) rather than feet. Will this cause me problems if I continue to
> re-import into the new location and integrate all maps?

No. x_0 is always specified in metres (technically, it's specified in
the same units as the ellipsoid, which is invariably metres).

Conversion between feet and metres is the first step on input and the
last step on output. Internally, everything uses metres, so projection
parameters such as x_0 use metres.

>    After running 'g.region vect=basins' and 'g.region -p' I see:
> 
> projection: 99 (Lambert Conformal Conic)
> zone:       0
> datum:      nad83
> ellipsoid:  grs80
> north:      1683494.51666667
> south:      4017.2
> west:       222006.05
> east:       2367279.85
> nsres:      1.00000019
> ewres:      0.99999991
> rows:       1679477
> cols:       2145274
> cells:      3602938341698
> 
>    The rows, cols, and cells seem extra large.

Well, you have an area of around 318 x 406 miles at a resolution of
one foot.

> This is a vector map so it probably doesn't matter,

It doesn't.

> but I'll be re-importing the DEM where it will matter.

The importation won't be affected by the resolution (or the bounds),
but any processing will be.

> Should I also clean up the resolution so both values are exactly
> 1.0?

The rows and cols values have to be integers, so if you want the
resolution to integers (or at least multiples or fractions of a
specific value), you have to adjust the bounds (e.g. with
"g.region -a").

If you're dealing with a single raster map, it's often easier to just
set the region to match the map's native grid with g.region rast=...

-- 
Glynn Clements <glynn at gclements.plus.com>