[GRASSLIST:8213] problems in gnomonic projection

Hello all,

I have some difficulty with transformation from/to gnomonic projection which I found in 'my' area and repeated in Spearfish training dataset.

For example, I set up a new location spearfish_gnom with following parameters (for faster calculation I chose coarser resolution than orig. Spearfish dataset):

$cat PROJ_INFO:
name: Gnomonic
proj: gnom
a: 6379000.0000000000
es: 0.0
f: 0.0
lat_0: 44.4600000000
lon_0: -103.8500000000

$cat DEFAULT_WIND:
proj: 99
zone: 0
north: 5000
south: -30000
east: 20000
west: -2000
cols: 220
rows: 350
e-w resol: 100
n-s resol: 100
top: 1
bottom: 0
cols3: 1
rows3: 1
depths: 1
e-w resol3: 1
n-s resol3: 1
t-b resol: 1

I reprojected elevation.dem from spearfish to spearfish_gnom and found that there is an evident shift in NS (y) direction of about 20 minutes (20'). EW direction is OK. The same results were on other area in Europe and were the same when using v.proj.

In case the problem is between the keyboard and my chair I put the r.proj command:

GRASS 6.0.1 (spearfish_gnom):~ > r.proj input=elevation.dem location=spearfish mapset=PERMANENT output=elevation

I work on Mandrake 10.1, proj 4.4.9. The proj itself converts the projections OK.

Do you have any suggestion?

Best regards

Milan Salek

Milan Salek wrote:

$cat PROJ_INFO:
name: Gnomonic
proj: gnom
a: 6379000.0000000000
es: 0.0
f: 0.0
lat_0: 44.4600000000
lon_0: -103.8500000000

$cat DEFAULT_WIND:
proj: 99
zone: 0
north: 5000
south: -30000
east: 20000
west: -2000
cols: 220
rows: 350
e-w resol: 100
n-s resol: 100
top: 1
bottom: 0
cols3: 1
rows3: 1
depths: 1
e-w resol3: 1
n-s resol3: 1
t-b resol: 1

I reprojected elevation.dem from spearfish to spearfish_gnom and found that there is an evident shift in NS (y) direction of about 20 minutes (20'). EW direction is OK. The same results were on other area in Europe and were the same when using v.proj.

Shifted NS compared to what? Why are you expressing the shift in degrees though both spearfish60 (utm) and your gnom location are using meter units?

You use a spherical earth model in your PROJ_INFO. That could explain why you see unexpected NS values while EW is OK.

rgds
Morten