GPS III + Maps
clark dong
This is a newbie question....
I'm interested in touring (both car and on foot) the major cities in
Europe with a Garmin GPS III. Since the US version of G3 does not have
much in the foreign map area, I suppose the next best alternative is to
use a paper map with lat+lon (using a laptop is too bulky). The perfect
solution would be to download custom maps to the G3, but, whom am I
kidding?
The question then is "Is it possible to purchase lat+lon maps for say,
London or Paris?". The last time I was on a walking tour in Rome I got
so completely disoriented by the
thousand-year-old-non-orthogonal-poorly-marked streets that I vowed to
get some hi-tech help the next time.
I'm also very interested in anyone who has done this with a GPS before
and any pointers that you like to share.
-cd
Christopher Davis
cd> == clark dong <cla...@hybrid.com>
cd> The question then is "Is it possible to purchase lat+lon maps for
cd> say, London or Paris?".
I can't help you with Paris, but for London and most other UK cities I
can give you some advice. Buy one of the "A-Z" map books for the area
you'll be in, making sure it includes the "Ordnance Survey" grid lines
(London A-Z does, Coventry A-Z does, the A-Z road atlas does, the
Birmingham one didn't last year when I checked).
Set your (Garmin, though some other mfrs support OSGB too) GPS to read
out in "British Grid" format *AND SET THE DATUM TO Ord Srvy GB* (or
you will be sorry). You can then use the grid lines on the maps to
figure out exactly where you are.
The OSGB grid readouts on your GPS will have a two-letter "square code"
(which you can mostly ignore when on foot, you won't cross grid square
lines that often), then an "easting" value, then a "northing" value on
the next line. ("Read over, then up" is the rule to remember.)
The A-Z maps will have grid refs in all numbers; the first digit of each
of the northings and eastings will be printed smaller than the others
and corresponds to the two-letter "square code" (again, this will only
be likely to matter in the atlas).
For example, if your Garmin is reading
SP 38856
BNG 81958
that is translated as
SP - grid square, see below
38856 - easting (except for 100km digit, which is 4 in square SP)
BNG - British National Grid (output format)
81958 - northing (except for 100km digit, 2 in square SP)
The numeric form of this coordinate would be 438856 281958. This
location is actually the parking lot of "Mega Bowl" in Walsgrave,
Coventry (I marked a waypoint there and had it handy, what can I say).
<URL:http://www.streetmap.co.uk/> is misbehaving today or I'd have a
better example to give you (like, actually in London).
Of course, few maps are going to have meter resolution, so what you'll
wind up with is a grid on the London map that has "71", three unmarked
lines, then "72" (for eastings). In that case the "71" line is "71000"
easting in that grid square, and you can interpolate from there; the
first unmarked line is "71250" easting, etc.
With practice you can do this conversion easily in a moving car
(assuming you're not the driver obviously :-).
There's a cute Shockwave gimmick on the Ordnance Survey web site about
the grid: <URL:http://www.ordsvy.govt.uk/products/natgrid/index.html>.
[Many thanks to Mike Green for the explanation and table (below) of how
OSGB letter codes mapped to the A-Z numeric-only grids.]
================================================================
KEY TO UK NATIONAL GRID ALPHABETICAL PREFIXES
The UK national grid is based on 100 kilometre squares, each given a
two-letter code which appears on Ordnance Survey maps. The grid origin
is a point about 80 km W of the Scilly Islands, and the table below
gives the distance in kilometres of the South West corner of each square
from this origin. This key will enable distances in kilometres to be
calculated from grid references using the two-letter prefixes, by
converting to Km and using Pythagoras. The hundreds digit of kilometres
is given in the corners of the OS 1:250,000 and 1:50,000 maps and can be
used for plotting grid refs instead of the two letter references.
GRID SQUARE (alph order) SW CORNER OF SQUARE East North
NB 100 900 SJ 300 300
NC 200 900 SK 400 300
ND 300 900 SM 100 200
NF 0 800 SN 200 200
NG 100 800 SO 300 200
NH 200 800 SP 400 200
NJ 300 800 SR 100 100
NK 400 800 SS 200 100
NM 100 700 ST 300 100
NN 200 700 SU 400 100
NO 300 700 SW 100 0
NR 100 600 SX 200 0
NS 200 600 SY 300 0
NT 300 600 SZ 400 0
NU 400 600 TA 500 400
NW 100 500 TF 500 300
NX 200 500 TG 600 300
NY 300 500 TL 500 200
NZ 400 500 TM 600 200
SD 300 400 TQ 500 100
SE 400 400 TR 600 100
SH 200 300 TV 500 0
If you find a small 4 and a small 3 by the 00 lines on the map you're
looking at square 400, 300, i.e. SK. Input this to the Garmin as SK
nnnnn nnnnn
================================================================
--
Christopher Davis <c...@kei.com> <URL: http://www.kei.com/homepages/ckd/ >
Geographic locations in DNS! <URL: http://www.kei.com/homepages/ckd/dns-loc/ >