Special Stage Forums banner

1 - 3 of 3 Posts

·
Achievement Unlocked!
Joined
·
881 Posts
Discussion Starter #1
Last year I thought I would be "helpful" and I put LAT/LON in what is known as (DD) Degrees Decimal as well as (DMS) Degrees Minutes Seconds in documentation for High Desert Trails. Some of our route notes had (DDM) which is even more confusing - leftover from a previous capture of locations.
This is BAD for volunteers in the field.
Not only was the data confusing, it was hard for people to figure out what to type into their GPS units.
+42.36005 -71.056 -vs- 42° 21.603' N 71° 3.36' W -vs- 42°21'36.18"N 71°3'21.60"W

I'll particularly call out DDM which I think lends to A LOT of confusion: This format is sometimes referred to as just “GPS” and the data after the decimal is confused with decimal seconds, leading to errors in navigation. 3 input fields - to which people type in Degrees, Minutes, Seconds and end up in a very wrong location. When they really needed to type in Degrees, Minutes, and everything after the decimal point.

So - This year we came up with a GPS coordinate system standard:
http://highdeserttrails.com/downloads/HDT GPS Coordinate Standard.pdf

The idea is that any and all GPS coordinates that we publish will only be in (DMS) Degrees Minutes Seconds!
If you need them in any other format, you can derive in the field with division by 60.
We know that you can multiply by 60 to get DD back to DMS, but the majority of GPS units accept DMS.
The other thing is making people AWARE of these 3 systems of representing LAT/LON data.

Check it out!
- Kris
 

·
Premium Member
Joined
·
1,570 Posts
I ran somewhat afoul of this before, though not at a rally. A friend texted me GPS directions using his iPhone, to find a remote offroad campsite in the dark, which I received in Google's default format i.e. what you call Decimal Degrees. Luckily I was aware of the different formatting modes, but the GPS nav built into my stereo head unit only accepts input in the format of Degrees Minutes Seconds. Thankfully I had my handheld GPS on me, so I used my GPS nav unit in my stereo to get somewhat close (put in nearest town & road), and then used my handheld in Decimal Degrees mode to get the rest of the way.

Anyway, I just did some investigation & testing:

- Google Maps PC application will accept GPS coordinates in all 3 formats, providing correctly formatted
[in fact, at least with Google Maps PC, you can even use different GPS coordinate formats for the starting point & end point, provided both are correctly formatted. You can't mix formats within an end point though, so you can't input e.g. the longitude in one format, and the latitude in a different format, but that's hardly if ever likely to be useful, so no big deal]

- Google Maps Mobile application on my smartphone will also accept GPS coordinates in all 3 formats, providing correctly formatted

- My handheld GPS (Garmin 60CSx) will also accept GPS coordinates in all 3 formats, providing one selects the correct units formatting in the setup menu

- the GPS Nav system built into the head unit in my stereo will only accept GPS coordinates in Degrees Minutes Seconds format

- so as a result I am fine with DMS being the standard, as most of my units support all 3, with my GPS nav in my stereo being the limiting factor supporting only DMS

- one could make an argument that DD should be the preferred format, as most people are most likely to be printing instructions using Google Maps on their PC, or using the Google Maps app live on their smartphone, and DD.ddddd is Google's default format. However, Google does support DMS (though coordinates in DMS format are marginally more tricky to enter on a smartphone as you need to add a space after the degrees & access to the ' and " characters), and DMS is the more traditional format, so the DMS format is more likely to be supported by more devices (see my GPS stereo comment above, as an example).

Some comments:

While I am fine with your descriptions of the different coordinate systems, I would prefer to see the formatting shown as follows as it helps illustrate the difference:
DDDº MM' SS.ss" (Degrees, Minutes, Seconds, Hundredths of a Second)
DDDº MM.mmm' (Degrees, Minutes, Thousandths of a Minute)
DDD.dddddº (Degrees, Fractions of a Degree to 5 decimal places)

In your description for DDM, I would elaborate the "Confusingly this format is sometimes referred to as just “GPS” and the data after the decimal is confused with decimal seconds, leading to errors in navigation." sentence to illustrate the difference i.e. "Confusingly this format is sometimes referred to as just “GPS” and the data after the decimal is sometimes incorrectly interpreted to be decimal seconds, instead of fractions of a minute, leading to errors in navigation."

Similarly, in the description for DD, I would change the description of "This format expresses latitude and longitude in geographic coordinates as decimal fractions" to "This format expresses latitude and longitude in geographic coordinates as
degrees and decimal fractions of a degree" to be clearer.

I would move the "Confirm BEFORE the rally how your GPS device accepts data" caution from the DDM section to the top, since this caution applies to all formats.

I'd remove the "WGS84: World Geodetic System of 1984. A geocentric datum and geographic coordinate system
created by the United States military. Popular GPS devices and Google Maps / Google Earth use this." note, or perhaps move it up to under or next to the "The rally uses the WGS84 datum standard." note. The Datum system used and the formatting of the coordinates used are related, but separate issues.

Anyway, as you state, the main point is awareness. Being aware of both what format the GPS coordinates in the rally instructions & notes are given in, as well as what GPS coordinate formatting modes are supported by one's equipment, so one can actually get to the intended location.
 

·
Achievement Unlocked!
Joined
·
881 Posts
Discussion Starter #3
Hey Piers - Awesome work here.
Let me take some time to digest / edit it - and we'll get the document updated so it can be more widely adopted. I pretty much agree with everything you mentioned.
I'm glad that someone who really understood the material made some good comments. :) Thanks for taking the time.
- Kris
 
1 - 3 of 3 Posts
Top