GPS Decimal Conversation page

Post by **Chalk** » February 26th, 2006, 11:02 pm

I found this site tonight, handy site for converting decimal Lats/Longs....http://www.fcc.gov/mb/audio/bickel/DDDMMSS-decimal.html

dave7 · Post by **dave7** » February 27th, 2006, 7:50 am

I use this one quite a bit:

http://jeeep.com/details/coord/

Post by **Chalk** » February 27th, 2006, 8:13 am

Yes...That's the one I use mainly...just found this one...note the link is for NAD27...I'm still checking into it , but I think Maptech is NAD27, which will make your numbers off in most GPS's.

Post by **tin can** » February 27th, 2006, 8:24 am

Chalk wrote:Yes...That's the one I use mainly...just found this one...note the link is for NAD27...I'm still checking into it , but I think Maptech is NAD27, which will make your numbers off in most GPS's.

Mapsource, and your GPS, can be set to any of about 200 datum. You need to know what datum you're going to input into your GPS, and set the GPS to that datum before putting the coordinates in.

Post by **qoutrage** » February 27th, 2006, 8:26 am

Looks good to me. Bookmarked both- Now, if I can find em when I needum.

Post by **Chalk** » February 27th, 2006, 8:43 am

tin can wrote:Mapsource, and your GPS, can be set to any of about 200 datum. You need to know what datum you're going to input into your GPS, and set the GPS to that datum before putting the coordinates in.

I was stating that as if someone might think maptech is NAD83 and input those number, if you didn't know it was one or the other your numbers will be off....As I stated I think...but playing around last night with three different satellite sites, it appeared to be that way

Post by **tin can** » February 27th, 2006, 9:08 am

I was agreeing with you. I should add to my post that one must be aware of what datum the information you're using (Maptech, Terraserver...) is based on, and set your GPS accordingly.

I have also determined that if you're inputting numbers with your GPS set to NAD27, you need to set the GPS to NAD27 to navigate to those points.

That said, the default setting for most GPS units is WGS84 (NAD83 and WGS84 are virtually the same datum). Most of the GPS mapping software is based on WGS84. If you already have a bunch of points in your GPS based on WGS84, set your GPS to NAD27, and input coordinates based on NAD27, now you have points in your machine on both datum. There is roughly 1/4 mile difference in the two datum.

I could go on with this, but I'm sure I have confused things enough.

Post by **Sir reel** » February 27th, 2006, 9:36 am

I think most of us on this site understand that when it sounds like Chalk and TC are disagreeing.... they'r really agree'n

Post by **Eerman** » February 27th, 2006, 1:49 pm

Anybody ever suggest a NFGFC meeting about this topic...maybe someone from West Marine or the always knowledgable TC could school us on these finer points of GPS use??

Post by **Sir reel** » February 27th, 2006, 4:28 pm

here is a question for you all.

If you have numbers in one format and you change your GPS to that mode and enter them. Does your unit correctly convert them when you change the unit back to a different format?

CROCKER · Post by **CROCKER** » February 27th, 2006, 5:36 pm

JUST A THOUGHT, I HAVE DONE A LITTLE SURVEYING AND I COMMUNICATE WITH SEVERAL SURVEYORS. SURVEYING GRADE EQUIPMENT WILL GET YOU WITHIN MILLIMETERS OF THE TRUE POSITION ON CERTAIN DAYS DEPENDING ON THE LOCATION AND AVAILABILITY OF SATELITES. THE OTHER DAY I WAS TALKING TO A SURVEYOR AND HIS POSITION WAS 4' OFF BECAUSE OF BAD SATELITE CONFIGURATIONS.

IF HIS EQUIPMENT FROM GERMANY AND SWITZERLAND COST $50,000 AND IS OFF BY 4', HOW FAR OFF IS YOUR EQUIPMENT FROM CHINA THAT COSTS $400? ?????? 500' ?????? IS THAT NAD 27 OR WAS83

HOW COME THE LORAN SIGNALS ARE STILL BEING TRANSMITTED? MAY BE BECAUSE SOMEONE KNOWS HOW MUCH IT COSTS TO OBTAIN SUB METER ACCURACY.

GPS IS NICE BUT LORAN IS LOT MORE CONSISTANT.

Post by **tin can** » February 27th, 2006, 6:35 pm

A discussion on Datum would be very interesting.

I experimented with switching between WGS84 and NAD27 on the water. And I can't answer the question about what happens internally in a GPS.

Crocker, the $50,000 stuff gives you accuracy to +/- 0.005', if used properly. Your $400 unit, if WAAS enabled, gives you +/- 10' accuracy.

Now, lets discuss LORAN.

LORAN is not really that accurate. But it is repeatable. Radio signals from the LORAN towers get distorted by land masses, buildings, and other obstructions. That said, if you plot your TD's on a map, the point might not be exactly where you plot it. But, if the LORAN signal warpage is consistant, you can go back to that same bottom structure every time.
It also depends on where you are in the grid. LORAN is using an ARC/ARC intersection. The closer your point of intersection angle is to 90 degrees, the more accurate the reading. As that point of intersection angle widens or narrows, the accuracy decreases. Also, GPS updates much faster than LORAN. You can be well past your point before LORAN updates to tell you you've passed the point.

LORAN is still active because the Government made a commitment to keep it active until, I think, 2008. After that commitment is met, if a LORAN tower goes down, it will not be repaired.

I'll keep my GPS.

Post by **Chalk** » February 27th, 2006, 6:37 pm

That's why we get concerned with what datum we are using....if you input NAD27 into a NAD83, your gonna be twice as much off...My stuff from China has been precise enough for me.

LORAN is on the table to be axed from the Homeland Security Bill last I heard, probably not being used 2% of what it was 10 years ago and cost several million dollars a year to keep up.

Post by **Chalk** » February 27th, 2006, 8:51 pm

All this highly sophisticated talk of datum, intersection angles and Chinese GPS and I can't spell decimal in the title....

Post by **Jumptrout51** » February 27th, 2006, 9:30 pm

If your machine is like mine it converts them automatically.