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GPS stuff in here?


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#1
tact

Posted 06 April 2005 - 02:54 PM

tact

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Hi all,

Since a lot of GPS talk in the "lets go offroad" thought to ease the
clutter and start a new thread.

keewes337 you asked about a GPS recommendation. I still have not sorted
the features out yet... but here is a good page to start with..



#2
GoToy

Posted 06 April 2005 - 03:56 PM

GoToy

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I am contemplating the purchase of a new GPS, Garmin Map76 to be exact.
The price is 1500rm but what attracts me to it is it`s almost guarenteed
ability to be able to take reading under the canopy. Theres cheaper on
the market, but I don`t want to be in the same position as I am in
now...a GPS that takes forever to take a coordinate.

I have been using GPS for more than 15years now and I still say that they
are just an aide to good map and compass work. The accuracy of GPS has
inproved tremendously over the years, but you will still only have an
accuracy of within 100m (I know people will say 3m, can be achieved I
know!). However, now is 2005 and with the wide availability of GPS most
people now will make them there prime navigation aide. I also have a
compass in my watch, but its not accuarate to navigate, a quality field
compass beats a watch and even a GPS!!

#3
keewes337

Posted 06 April 2005 - 05:34 PM

keewes337

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Just to repost some links

PC Map editor and reader - http://www.geopainting.com/en/

Map resource - http://mapcenter.cgpsmapper.com/

tact - thanks for the link will look into it.



#4
tact

Posted 06 April 2005 - 06:24 PM

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I`d disagree GoToy. A compass is a device relying on magnetic fields and
these are not constant. even magnetic materials can cause a compass to
indicate incorrectly.

magnetic declination changes annually, and from area to area. and even if
you KNOW the supposed magnetic declination for the map you are working on
it is a generalisation. ie. that within the borders of the map there may
well be many uncharted magnetic disturbances.

A GPS however is not subjected to such variance. Yes there is a positional
variation possible up to 100M (tho less (supposedly 15M) since the US gov`t
turned OFF selective availability in year 2000).

An accuracy of +-15M for the departure and destination locations on the
ground is insignificant even over 10,000km

but an error of a fraction of a degree in a compass due to local uncharted
magnetic anomaly, when extended over enough distance, can mean you may miss
your destination by many many kilometers

a GPS will always point to the destination waypoint no matter where you are
in relation to the destination waypoint, no matter how far you are away,
with an accuracy +- 15M (and if SA were still active, within 100M but
thats far less than kilometers!)

The problem is that people who have not used GPS are unaware of things
like... you MUST be moving before any direction, bearing, etc, can be
resolved. So a compass helps you move in the right direction for the
first few steps. after that the GPS has oriented pointing you right.

I began my career in navigating with map and compass 20-30yrs ago and used
the skills regularly since then. Since I started navigating with GPS only
8yrs ago - the compass hardly comes out (except when planning routes on a
table and then a protractor would do as good a job).


#5
cvlchin

Posted 06 April 2005 - 07:10 PM

cvlchin

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Hi all,

GPS is certainly a much welcome navaid. As is the compass, a GPS is also
with its own limitations.

For accuracy within 100 meters, a GPS would need to have corrections
done.What is needed to make this type of accuracy is of course the much
used DGPS. With the differentials (the D of it) correction the accuracy
will be much more defined. And of course the map type used should also be
taken into account.

So if there's the need for accuracy and differential corrections is not
within means, one would still need map reading skills, particulary in the
rainforest.



#6
keewes337

Posted 06 April 2005 - 07:54 PM

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What about this 'WAAS'? Is this available here?

Do you guys know a place/shop in KL where you can actualy test/play with
various gps units before actually buying it? I've been to Low Yat and all
they have are Garmins.


#7
tact

Posted 06 April 2005 - 08:11 PM

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Hi cvlchin,

you wrote:
"For accuracy within 100 meters, a GPS would need to have corrections
done.What is needed to make this type of accuracy is of course the much
used DGPS. "
======

Have been doing some reading to update my knowledge (last study I did into
GPS technology was when I bought my first in 1997) and apparently Selective
Availability (SA) has been turned off since May 2000.

Apparently nowadays (SA turned off) people think of standard GPS accuracy
(no corrections needed) in terms 25M, not 100M.

With DGPS that can drop to 15M, and in some cases less than 1 metre.

Accuracy in perspective:
on a 1:100,000 map - 1cm = 1000M
ie. 1mm = 100M

on a 1:50,000 map - 2cm = 1000M
ie. 1mm = 50M

on a 1:25,000 map - 4cm = 1000M
i.e 1mm = 25M

Is it fair to say that the pencil line one might draw on a map is maybe 1mm
wide?

That one millimetre wide pencil line is "25M wide" on a 1:25000 map
it is 50M wide on a 1:50,000 map
it is 100M wide on a 1:100,000 map.

Makes the generally accepted "25M" figure for GPS average accuracy look
pretty good.



#8
tact

Posted 06 April 2005 - 08:19 PM

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don't know about WAAS in malaysia. its mentioned in this article. another
good recommendation i think...



#9
GoToy

Posted 06 April 2005 - 09:06 PM

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A filed compass using the Nato mils 6400 is far more accurate than a GPS.
You physicaly can`t match the 6400 mils of a compass on an lcd screen!!.
If your good with a compass you`ll have accuracy down to 10m.

The US military has not turned off the the random blips on the NAV
satellites.

There is no way can you match a 6400mil compass with a lcd screen. Yes a
GPS will give you the position, but for following...NO cannot be more
accurate.

Another rather nice GPS is the Garmin GPSMap60CS, I had a look at one
this evening...very nice....and a not so nice price too!!RPLF.

#10
tact

Posted 06 April 2005 - 10:01 PM

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apples and oranges GoToy.

GPS methodology and map/compass methodology are different. the concept of
"accuracy" when comparing the two gets very blurry. does not matter how
accurate your compass is, or how blurred an LCD screen.

-Compass points north.

-GPS points to the destination. (or a place notionally within 25M of it
regardless how far you travel to get to it).

-Compass... no matter how accurate your tools, you can miss the destination
point and not know it. (bearing calculation errors, errors taking bearings
on the move, crosstrack errors). And these errors compound the longer the
bearing is (not a constant approx 25M). Compass points North.

-GPS... you just cannot miss the spot on the earth that is the nominated
destination. GPS points to the destination.