Welcome, Guest. Please login or register.

Login with username, password and session length
Forums Search:  


Author Topic: Garmin TOPO US 100K errors in trail location  (Read 3943 times)

Hiker02093

  • Newbie
  • *
  • Posts: 1
  • Karma: 0
    • View Profile
Garmin TOPO US 100K errors in trail location
« on: October 28, 2016, 06:52:47 AM »
I'm new to using a land based GPS, I'm a converted sailor and have used Garmin marine GPS units and they have always been really good.
However, I recently purchased an eTrex30 with TOPO 100K. While getting used to the unit and the Base Camp software I noticed that the representation of a trail(Cascade Link,Mount Monadnock) was off quite a bit. I wrote Garmin support, questioning as to why and if that view was unique to Base Camp or would I see the same on the trail. They basically said that's what I get and suggested buying TOPO US 24K. I'm not confident that it will be more accurate.
Can some more experienced hikers please weigh in?

Boyd

  • Expert Advisor
  • *****
  • Posts: 4025
  • Karma: 50
    • View Profile
Re: Garmin TOPO US 100K errors in trail location
« Reply #1 on: October 28, 2016, 07:22:07 AM »
Are you familiar with the old paper USGS 1:100,000 maps? They were also wildly inaccurate. They were handy because they covered a large area, but the detail and accuracy was nowhere near the old paper 1:24,000 maps. It would take sixteen 24k maps to cover the same area as one 100k map in fact.

Digital maps can't really be compared to paper, but I think Garmin makes the assumption that users will understand that a 100k map isn't intended to be terribly accurate. Another way to compare this is that the Garmin 100k map of the entire US is about 3gb. However each of their 24k topo maps (which only cover a few states) is about that same size.

I have owned a few different versions of the Garmin 100k maps over the years (they used to call it "Topo US" before they introduced the 24k map series. It was not at all unusual to find errors of hundreds of feet in the location of roads. I realize you're new to this, but the inaccuracies of the Garmin 100k maps are well known. My understanding is that they have actually improved recently as Garmin switched to HERE (formerly called Navteq) as a source of road data.

Regarding trails, I wouldn't blindly accept that the 24k map will be better. Roads and other details will be more accurate, but you might find that the trails aren't there at all. Many people have complained that the 24k maps have fewer trails than the 100k maps.

There are a number of free maps here that include trails, some of them may be better than the Garmin map. Why not give them a try before buying anything else? There are also sites where you can download tracks, and I believe your eTrex has the advanced track navigation feature that allows you to use a track like a trail.

maps4gps

  • Expert Advisor
  • *****
  • Posts: 1532
  • Karma: 18
    • View Profile
Re: Garmin TOPO US 100K errors in trail location
« Reply #2 on: October 29, 2016, 03:39:04 PM »
I would like to try to clarify some ' :)wildly inaccurate' statements/impressions often made/held about map accuracy.

All federal government maps for standard or principal needs have to meet horizontal and vertical accuracy criteria/tests.  'For maps on publication scales' ... of 1:20,000 or smaller (ie. 1:24k, 1:63,360, 1:100k, 1:250k, etc.) 'not more than 10% of the points tested shall be in error by more than 1/50 inch' - that is 40 feet on a 1:24k map and 167 feet on a 1:100k map.
Furthermore 'these limits of accuracy shall apply in all cases to positions of well-defined points only.  ....'  [I was once told by a USGS cartographer in Reston that the thinnest line that could be printed on paper was 1/100 inch]

Would not be much reason to use 32 sheets of paper for 1:24k's if they did not show more detail than a 1 sheet 1:100k.   Except in special cases like a small over-edge area, the 1:100k's covered an area of 30 x 60 minutes; the 1:24k's were 7-1/2 by 7-12 minutes.  There are edge and central tic marks showing the 4 by 8 array of 1:24k's.  FYI - the 1:100k data was created by paneling the 32 1:24k maps.  I never got a detailed explanation from USGS of the process; however, the 1:24k maps needed to be created first - this required the creation of those 20,000+ preliminary version (only a few of which went to final/standard maps).

No Assumption needed - a 1:24k has 4+ (100,000 / 24,000) by 4+ times the paper area available to show the same area of the ground surface and it's various features.  IIRC at the time Garmin released the 1:100k mapset the only other topo product they were selling was 1:24,000 mapsets of individual National Parks on 3 CDROMS sold separately.

Only the hydro and transportation data was digitized (for the Bureau of Census).  Garmin must have digitized the other types of features themselves or found other data sources.  Boyd or anyone; have you ever over-layed the Garmin data on the USGS 1:100k DRG (digital raster graphic) images of the same area or compared it with the USGS 1:100k DLG (digital line graphs)?  The USGS DLG files were in NAD27 and UTM coordinates - perhaps there was a glitch in the conversion process - I initially had some using GM when I created the Trails from USGS 1:100k overlay mapset.
« Last Edit: October 29, 2016, 04:39:51 PM by maps4gps »

Boyd

  • Expert Advisor
  • *****
  • Posts: 4025
  • Karma: 50
    • View Profile
Re: Garmin TOPO US 100K errors in trail location
« Reply #3 on: October 29, 2016, 04:37:28 PM »
a 1:24k has 4+ (100,000 / 24,000) by 4+ times the paper area available to show the same area of the ground surface and it's various features.

Boyd or anyone; have you ever over-layed the Garmin data on the USGS 1:100k DRG (digital raster graphic) images of the same area or compared it with the USGS 1:100k DLG (digital line graphs)?

I think we're saying the same thing… 24 x 4 = 96 which is "close enough" to 100 for our purposes. :) Since area is a squared function, a 100k paper map covers about 16 times (4x4) the area of a single 24k map.

I have not overlaid the Garmin 100k maps, but used them starting in 2004, updated them in 2008 and bought an Oregon 400t which was the first device that Garmin pre-installed the 100k topo map on. That was the only Garmin map covering my area for awhile, until they introduced the 24k series. I would not buy one of the "T models" today (handhelds with preinstalled 100k maps).

The errors were obvious when driving around and seeing the road hundreds of feet away. But these older versions were based on TIGER data, and I saw the same kinds of errors when I used that data to make my own maps back then. A lot of it has propogated into OSM as well.

But two or three years ago, Garmin switched to using Navteq/HERE road data. I have not seen these new 100k maps, but people report they're much better.

Back before I had GPS (stone age), this 100k map was my hiking companion. Detail was certainly lacking, but it covered a large area and folded up to fit my pocket.  8)

« Last Edit: October 29, 2016, 04:41:54 PM by Boyd »

 

anything