GPS coordinates

L

Larry Patrick

New member
I need to look up how to put waypoints in Garmin 4208. Will read manual,as im reading about areas warning of obstructions on Lake Erie they give you Lat and Long. Also some fishing maps also.Always to busy on boat trips to sit down and read it,so I will dig it out and research it,guess its not difficult? Hitting the man overboard to mark fishing spots is as far as I got.
 
You go to mark waypoint. You will have a # for that waypoint. At any time, you can put in your "code" for that waypoint, or say "fishing hole at silver point"--or what you want. The latter takes time, I prefer to just write in the log book what waypoint #4 is. MOB, will mark a MOB way point.

Here is what Garmin Says about convention of how you can enter manual waypoints:

While Garmin can display and convert from the three different systems used to define lat/lon there can be considerable confusion among users looking at the display and comparing it to some external information. This is because some data is not very precise in its use of decimal points and the lack of a degree sign on most keyboards can encourage a substitution of the decimal point. When comparing numbers consider that minutes can only go to 59 and will then roll over. If the data just after the decimal has digits above 5 then it is likely to be a decimal part of a degree and not minutes. Similarly for seconds.

Decimal parts of a degree can be converted to minutes easily by multiplying by 60. A space between degrees and minutes is the preferred separator when a degree sign cannot be used to avoid confusion. Similarly decimal parts of a minute can be converted to seconds by multiplying by 60 thus, if you have ddd.ddddd you can convert to ddd mm.mmm by:

ddd mm.mmm = ddd + (0.ddddd x 60)
or conversely
ddd.ddddd = ddd + (mm.mmm / 60)
Click to expand...

Also be sure that your Garmin is set to WGS 84--which is their default Datum.
 
This situation is a puzzle to me. It seems that Coast Guard, NPS and other organizations protecting the waterways and people would require all GPS to have a single default mode of stating lat and lon. Apparently not, as the differences have, according to news coverage, has cost lives and money.
 
I'm not where I can get to it now but a couple of years ago BoatUS magazine had an article about GPS Latin and Long designation and depending on which one you report there could be as much as a half mile difference between the three ways to record and report a location.

Harvey
SleepyC :moon

:embarrased :oops:
 
Harvey;

That's interesting. I assume you mean using decimal degrees versus degrees, minutes and seconds versus degrees and decimal minutes. Given the same precision any change between those units are mathematical and they should be equivalent.

Could you elaborate further? I did a search and couldn't find it.

Boris
 
My understanding is that the problem was with the person either reporting, or interrupting the location. Borris is correct. But the lack of a marine/land/SAR convention is confusing. Often co-ordinates are given to a dispatcher, and they don't understand, or ask the person reporting to clarify how units are used. Heck most boaters don't understand the difference! A few years ago we had a seminar for first responders, the 911 operators, Dispatchers, various charter boat and dive boat skippers, Coast Guard etc, so that everyone in the 3 county area was all on the same page. But that does not mean that the average recreational boater knows beans about the various ways to determine location....

Throw in the UTM and the (MGRS) Military Grid Reference System, it really gets confusion, plus as I noted above, you have to be using the same datum, (basically a survey point system).
 
Thanks for good information. We are working on natural gas line jobs quite a bit marcelus shale UGI jobs, lately,harrisburg,lancaster ,Tunkhannock ect about 25 different locations a season. Plus or usual work .We do the concrete work for whatever is needed. Most of the time we are given gps coordinates, we punch them into garmin car gps,fairly easy. That's what made me think about using coordinates on the water, for known fishing hot spots on maps ect.
 
Nearly any consumer GPS unit will feature different formats for coordinate display. Different brands will have similar but not the same defaults in format and can also vary in the number of decimal places shown which can cause confusion. Also, communicating a position over a radio can be challenging both to decifer and to recognize the format correctly. This can lead to all types of terrible errors and issues in response. When teaching regional SAR teams in my past life, I tried my best to standardize our use on Decimal Degrees which is the most straight forward to write, speak, and not make mistakes.

The default datum used for most consumer units will be WGS-84. If the person providing a position does not report a datum, or does not know which datum is used, assume it is the WGS-84. Also assume the NAD-83 to be the same for navigational purposes. If however, you have a person (more likely on land) report a position using the NAD-27 datum, you can assume it will be no more than 300ft in error compared to the NAD-83 and WGS-84 in the lower 48 with the west coast being the worst.

Your GPS is a great coordinate/datum converter so take any transmission down on paper first. Then you have the option of your changing your coordinate/datum settings to match the input coordinates, then switching your setting back to another format which converts any data in the unit to now display that current format.

Of the three coordinate formats below, which looks the simplest to you? Having a written example of each format in front of a radio operator is one of the easiest methods of matching an incoming transmission with a given format and not making a mistake. Counting digits is a good basic triage method but does not work 100% of the time due to decimal variance of different brands. (getting rare)

Degrees Minutes Seconds 36°14'55.35" N 96°36'53.95" W
Degrees Minutes 36°14.9226 -96°36.89928
Decimal Degrees 36.24871 -96.614988

If you have any points of confusion or have further questions about waypoint use for your GPS units, I would first refer you to the owners manual for direction, then come back here and ask. Someone will help!

More datum details of you care to dig further....

Many folks think the lat/lon measuring system is independent of the datum issue however this is not the case. In earlier times, measurements weren't precise enough to make this obvious. Even today, making angular measurements using devices like sextants is not accurate enough to make the datum usage much of an issue.

Translations among the various datums can be a cause of some errors. Garmin uses the Molodensky transform parameters for those datums and performs transformations as needed. This is a simplified model and can result in moderate errors. With this transform the WGS-84 and NAD-83 numbers are always the same while a mapping grade translation will show a slight difference in these two datums. For most of the US this difference is less than a meter rising to about 2 meters on the west coast. Still less difference than the accuracy of the unit in most cases. Modeling something as complicated as the earth does not lend itself to mathematical transformations so to achieve accurate results some information is contained in tables with model information used to access the entry in the table which is then interpolated to get the answer. It is not known to me (proprietary) where Garmin depends on models alone and where they supplement with tables.

WGS-84 is a world wide datum and is the master datum for use with a gps. Garmin always stores internal system information in this datum. The origin is the center of the earth and then a ellipse is defined using the major and minor axis. Information about ellipse flattening and the gravitational constant is also part of the definition. The latter is used to calculate the geoid height. The model is augmented with stations at precisely defined locations on the earth's surface to pinpoint the accuracy of this system.

Many existing older maps use the NAD27 datum. The difference in coordinates between NAD27 and NAD83/WGS-84 can vary up to 10's of meters. The 1927 datum was determined from less accurate and fewer observations. Factors such as seismic motion have changed station positions over time and computational capabilities did not exist to average readings. The newer datums are geocentric while this datum was based on a point in Kansas, and gravity data was not used in the network adjustment of the 1927 datum.

Greg
 
Excellent post by Greg, who is a professional in cartography. I guess when you ask what is the easiest position to understand, for me it is degrees, minutes and seconds. The reason is that I still used paper charts and on occasion I plot them on a chart. It is far easier to plot on a paper chart, using the lat and Long scales, using dividers and the marking on the margin of every chart. For me, translating each time a number is converted is a potential for error.
 
Thanks Greg!

Maybe someday a circle will have 100 degrees, a day 10 hours, an hour 100 minutes, and a minute 100 seconds. How much is a rod? A stone? A hand? :?
 
They tried those units (100 division in a circle, etc.) in the French Revolution. Even with cutting off people's heads they couldn't get it to work.

As to the equivalence of decimal degrees, degrees, decimal minute, etc, from Greg's post the error is not in the transformation but transmission and recording the position. So any of those formats is equal if we, as users only agree.

Boris
 
Thats right Roger, they do use that format but I am not sure of the history or reasoning. In the case of personal emergency, the Coast guard has the knowledge/tools to accept various formats so anyone can use what works well for them. If you would like to take a coordinate from the coast guard, then you can bet it would be in the degrees minutes format. If you want to be able to take a coordinate reported from another boat in distress, the format will vary. Best to keep pen and paper ready and learn your settings.

Be sure to start practicing the USCG standard reporting format so it sounds official.

Standard Latitude/Longitude
format for CISAR operations

The standard Latitude/Longitude format for
CISAR operations is Degrees, Decimal
Minutes (DD° MM.mm’).
Latitude is always read and written first
noting “North” since the U.S. is North of the
Equator.

Longitude is always read and
written last noting “West” since the U.S. is
West of the Prime Meridian.

When speaking Latitude and Longitude
coordinates for 39° 36.06’N by 76°
51.42’W. Latitude and longitude is stated
as:

“Three nine degrees, three six decimal zero
six minutes North by seven six degrees,
five one decimal four two minutes West.”
The words, “degrees,” “minutes,” and
“decimal” must to be spoken.

:)
 
Great explanation Greg. Also consider that if aircraft are part of the rescue, they will be using Degrees, Minutes, decimal minutes with WGS84 datum. Ken
 
Boris, I have been looking for that article, but have not found it yet. I'll keep trying. I think Greg has addressed much of what was in the article.

Greg. in the second (Degrees and Minutes) and third example (Decimal Degrees), Are North and West not included in the designation?

Degrees Minutes Seconds 36°14'55.35" N 96°36'53.95" W
Degrees Minutes 36°14.9226(?) -96°36.89928(?)
Decimal Degrees 36.24871(?) -96.614988(?)


Harvey
SleepyC :moon

0_CD_Cover_SlpyC_with_Classics_MBSP_2009_288.thumb.jpg
 
If ever hearing a distress signal,would be nice to feel confident in knowing how to use electronics to possibly be able to assist. Will practice when boat is on the water this season,been reading owners manual for Garmin.
 
I can answer this one:

Greg. in the second (Degrees and Minutes) and third example (Decimal Degrees), Are North and West not included in the designation?

Degrees Minutes Seconds 36°14'55.35" N 96°36'53.95" W
Degrees Minutes 36°14.9226(?) -96°36.89928(?)
Decimal Degrees 36.24871(?) -96.614988(?)

In gpx (GPs eXchange format) degrees are negative west of Greenwich Meridian, positive east, negative south of the equator, positive north, so the N/E/S/W indications aren't necessary, being implied by the positive or negative signs.
 
Perfect answer.

The negative is also not necessary for local use of any flavor of lat long. If we lived near the prime meridian or near the equator, we would have to be more careful. If we were gathering and plotting locations on a global scale, it would be important of course.

Greg
 
Thanks for the follow-up. I don't think I have ever heard a "Minus" or "Negative" included in a transmission of a Lat/Long.

Speaking to the writing down a position report and electronics practice.

Before last summer I had an ICOM M-506 VHF installed. It was chosen for 2 functions not on many VHF's. #1. It has an AIS receiver onbard, adding another degree of redundancy. AND #2. And the primary reason. It has a 2 minute, last call recorder built in. I use this radio strictly for monitoring VHF channel 16 now. It can hear much faster than I can write, and it is always ready to record that call. It is a quick 1 or 2 button push to get the playback.

Now back to the standard thread content. This is a good thread to know, practice and keep some note handy.

Harvey
SleepyC :moon

JC_Lately_SleepyC_Flat_Blue_070.thumb.jpg
 
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