[GRASSLIST:5808] Re: GRASS Novice Needs Help

[Tom Russo]

On Wed, Feb 16, 2005 at 10:01:54AM +0000, we recorded a bogon-computron collision of the <jronan at tssg.org> flavor, containing:
> Ok, something funny going on here thats the output from g.region -p 
> from both maps individually
> 
> GRASS 5.7.0:~ > g.region -p
> projection: 99 (Transverse Mercator)
> zone:       0
> datum:      ire65
> ellipsoid:  modif_airy
> north:      52.33030351
> south:      52.14872735
> west:       -7.41558677
> east:       -7.11971255
> nsres:      0.00003844
> ewres:      0.00006263
> rows:       4724
> cols:       4724
[...]
> Which doesn't at all agree with Tom's follow on email, Hmmm.
> 
> I suspect I've done something else stupid. When GRASS asked me for the 
> tie points for the rasters, I supplied the eastings northings in 
> latitude and longitude. Which I'm assuming now given that my 
> co-ordinate system is in Meters that this is completely wrong. That 
> wouldn't explain how they seem to display correctly though.

Yes, you need to give your tie-points in the northings and eastings of the
Irish Grid System which are in meters.

> Tom, how did you work out the tie points in meters?

Your maps have a grid that show the northings and eastings --- but obfuscated
a little.  Just as the US Military abbreviates UTMs by turning the first
few digits of each coordinate into a 2-letter grid square, and compresses
the remaining digits into a single 6-digit code, the Irish Grid system 
abbreviates the 6-digit northings and eastings into a single letter and two
3-digit partial coordinates.  Then on maps they throw the least significant
digit away when labeling the grid lines.

Your OSi maps have a 1000m grid with intersections marked as, for example,
 "41 11"
which is an abbreviation of the coordinates
 "S 410 110" 
(you told me that) as recognized by the Irish Grid calculator you first 
pointed me at.  With a little digging I determined that the "S" was an 
encoding of the 100Km digit of the northings and eastings, and that "410" and 
"110" are the 10k, 1k, and 100m digits of the coordinates.

In the case of the grid intersection 41/11 or abbreviated coordinate 
"S 410 110" the correct coordinates in meters are 241000, 111000.

For other lettered grids you'll have to work out what the first digits are
supposed to be, then you can map the 2-digit grid intersctions into 6 digit
eastings and northings quite easily.  It is these coordinates you need to use
to label tie points in i.points.  

To convert between known Irish Grid coordinates (in meters) to lat/lon
(for sanity-checking purposes), you can use the cs2cs command:

    cs2cs +proj=tmerc +lat_0=53.5000000000 +lon_0=-8.0000000000 \ 
        +k_0=1.0000350000 +x_0=200000.0000000000 +y_0=250000.0000000000 \
        +a=6377340.189 +rf=299.3249646 +no_defs \
        +towgs84=482.530,-130.596,564.557,-1.042,-0.214,-0.631,8.15 \
    +to +proj=latlong +datum=WGS84

To do it the other way, use:
    cs2cs +proj=latlong +datum=WGS84 \ 
     +to +proj=tmerc +lat_0=53.5000000000 +lon_0=-8.0000000000 \
        +k_0=1.0000350000 +x_0=200000.0000000000 +y_0=250000.0000000000 \
        +a=6377340.189 +rf=299.3249646 +no_defs \
        +towgs84=482.530,-130.596,564.557,-1.042,-0.214,-0.631,8.15 \

This is how I managed to get the fact that "S" meant "put a 2 in front of
the easting and a 1 in front of the northing" --- I used the converter web
site you told me about to convert S 410 110 into lat/lon, then used
cs2cs to convert it back into meters in Irish Grid.  When I say "sanity
checking" I mean double-checking in the final quality control step, e.g. plug
"241000 111000" into the first cs2cs command and you should find the 
lat/lon that puts you at the 41/11 grid intersection on the waterford/tramore
map I produced back in October.

Hope that helps.  Glad you posted here, because I was assuming you were 
using the northings and eastings in Irish Grid when I was trying to figure
out your other questions.

-- 
Tom Russo    KM5VY     SAR502  DM64ux         http://www.swcp.com/~russo/
Tijeras, NM  QRPL#1592 K2#398  SOC#236 AHTB#1 
 "When life gives you lemons, find someone with a paper cut."