Capt. George
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GARMIN MAPS
Does anyone use the Garmin Map #
MUS021R.....If so, does it offer good detail of Mulege through La Paz? Don't want the expense if it doesn't have the detail.
Appreciate your help.....Capt. George
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jk
Junior Nomad
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Registered: 8-23-2003
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Garmin maps
I have lots of Garmin maps including the one you referenced. My experience is the coastline detail is good, but the map is not accurately
georeferenced. By that I mean that all the waypoints of Mexico I have collected in the field do not accurately plot on the map I have in my computer.
They are all shifted generally about 1/4-1/2 mile east-northeast of their true location on the Garmin maps. That means the secret beach I always
camp on along the Sea of Cortez is mapped as if it is 1/2 mile offshore. I have spoken to Garmin about this problem several times and they say there
is nothing they can do. I say there is nothing they want to do (not enough customers to justify the expense), but I believe the solution is not that
complicated. It has to do with misidentifiation of something called a projection and that has produced the systematic shift of my waypoints (and I
assume everybody else's). I would love to hear from anyone who has plotted their field-collected waypoints from Mexico on Garmin maps and how
accurate their location is noted.
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Capt. George
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GARMIN MAPS
JK, thanks for the rapid and informative reply.....I will locate and reference my own waypoints (in due time). My concern is does it show a good deal
of bottom make up, or at least give me a confidence in where I can start....Here in the Keys, the maps actually will show one single coral
head....But, alas, money, money makes corp. America go round and round... Thank you JK Capt. George
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richard nauman
Junior Nomad
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Map Projections
Hey guys - I haven't used Garmin GPS or the maps you mention but do work with cartographic data a lot. Without going too nuts with detail the basic
problem map makers run into is that it's hard to "flatten" a round globe onto a flat map. Think about smashing an orange peel - it's got to rip or
fold to make a flat surface so what cartographers use is a mathematical conversion to make a globe flat - it always results in distortion in area,
direction or both. Different projections are used for different maps to minimize this distortion - it's all just a trade off. To make matters worse
the earth is not perfectly round. It is a little fat around the middle and flatter near the poles. There are also other irregularities in the shape.
So the projection that you use also needs what is called a "datum" which is an approxiamtion of the shape of the earth. If you're still reading here's
the point - if you put 2 maps together (like your way points and the Garmin Maps) with different projections or datums things will not line up
correctly. Judging from the distance and direction of your errors it sounds like you are not using the correct datum which is a common problem.
Generally GPSs come with WGS83 as the default datum (you should be able to find this in the set up menus). It is very likely that Garmin used USGS
data which is often in WGS24 - an older model of the earth. So if I am correct then you can reproject the Garmin map (hard for you to do) or change
the setting of the projection and datum in your GPS to match the map. THis won't help your old way points but should fix future ones.
Email me at RichardNauman at yahoo.com if you want more help or we can continue here if everyone is not alseep. The take home is that you need 4
things to mark a possition on a map: latitude, longitude, projection and datum - the second 2 are just as important as the first 2 especially if you
are trying to avoid those rocks in your boat!!!
RN
[Edited on 10-1-2003 by richard nauman]
[Edited on 10-1-2003 by richard nauman]
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jk
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Registered: 8-23-2003
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Hey Richard, thanks for the reply. I am aware of all the details you speak of (I supervise a GIS unit for work). The problem I speak of is not an
error in use of datum on my part, but may be an error in geoprojection of datum on Garmin's part. However, they are not willing to look into the
problem.
Capt. George, the depth info on the maps is perhaps less reliable than other features. If nothing else, depths are not sampled at any where near the
rate they are on US charts. I sure would not count on it to keep me out of trouble, even if there were not georeferencing problems.
[Edited on 10-2-2003 by jk]
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David K
Honored Nomad
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Location: San Diego County
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Thanks a bunch Richard... great info. The map datum pre set on my Garmin eTrex is WGS84.
For some more basic info. for GPS beginners: The map datum used on Mexican topo maps (incl. the 'Baja Almanac') is NAD27 (North American Datum of
1927).
The Garmin has all map datums stored in it, so it is easy to change from one datum to another. The waypoints (coordinates) automatically change when
the datum is switched.
In my many trip web pages, I provided some GPS waypoints. In the beginning, I just entered them as shown on the eTrex (WGS84). Now, with so many of us
using the Baja Almanac to spot these exotic locations and road junctions, I have posted the waypoints set at NAD27.
The important thing is noting the map datum and entering the waypoint with the GPS set at the same datum.
Neal Johns is my teacher... see his funny GPS article on the DesertExplores.org web site.
Baja Almanac's web site: http://www.baja-almanac.com
[Edited on 10-2-2003 by David K]
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Capt. George
Super Nomad
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GREAT
This site ia an almanac!
george
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richard nauman
Junior Nomad
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GPS, GIS and Baja
Hey guys I thought I was the only one who worried about this stuff (although I did mistype - David K is correct in that USGS maps are in NAD27). I
haven't used GPS much but I do a lot of GIS work. I didn't realize that the recreation grade GPS could reproject waypoints automatically. Sounds like
the Garmin map problem is a little deeper than I thought.
JK - do you know of any good sources for GIS layers for Baja? I just got ARCGIS at home and have been playing with the layers that ESRI provides and
that I downloaded from the web (mainly the coverages for the World atlas that ESRI produced). I'm just making maps for fun and always looking for Baja
Data. I found some elevation data but you need to buy the miltary analyst extension. I've been meaning to contact the almanac folks to see how they
made those great looking maps.
RN
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jk
Junior Nomad
Posts: 43
Registered: 8-23-2003
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Richard, I haven't really even thought of using ArcGIS with Baja data. Interesting idea--I'll have to see what I can find. I have limited myself to
the Garmin charts and I do have 1:250,000 topo maps of Baja (someone help me with the source, I can't remember off the top of my head). I used
Oziexplorer to work with them, but I haven't figured out a way yet to download them into a PDA. I really don't want to tote a laptop around Baja in
my sea kayak.
[Edited on 10-3-2003 by jk]
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Neal Johns
Super Nomad
Posts: 1687
Registered: 10-31-2002
Location: Lytle Creek, CA
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Mood: In love!
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jk,
Both OziExplorer and Fugawi work with PDAa now to use scanned maps.
My motto:
Never let a Dragon pass by without pulling its tail!
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jk
Junior Nomad
Posts: 43
Registered: 8-23-2003
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Neal, thanks for the tip. I will have to check out the upgrades. I just bought a Garmin iQue and would love to have those maps on it for my next
Baja trip.
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richard nauman
Junior Nomad
Posts: 67
Registered: 10-1-2003
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Mood: Tranquilo
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Baja GIS
JK - please let me know if you find any good GIS data - I'd like to make some simple maps to add to my web page.
RN
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