Lowrance Globalnav 12 Football Field Inaccuracy

[+Big_Lebowski]

Ive had this unit for about a year..

Its a Lowrance Globalnav 12

 

While it has served well for going out to the desert

and riding around (on unpaved roads only no offroading where I go)

I have always noticed this unit has seemed to be off by about

1 football field round...

 

This unit does have a Com port (cover came off not sure where its at now)

and due to family not beinf carefull it has some screen scratches nothing

too bad, but I wonder if there is a way to make this unit more accurate.

 

I have found in the manual and the unit it self where it mentions

DGPS correction, From my research this requires additional hardware..

I dont have the cash to buy a new unit, but would like to know if anyone

on here has any advice on how I can make this unit more accurate..

I would love to try and start finding geocache's etc and all but with this

current rate of inaccuracy I just feel it would be impossible and or

take HOURS of additional time to find a simple cache..

 

The software mentioned in this unit is as follows..

 

Copyright APR 8 1998

Version 3.1

250 Waypoints

 

please anyone able to help me in this thought of making the unit

more accurate.. I have tried a reset etc..

still seems to wander, gets a little better after sitting in one spot

for over a hour.

 

Thanks everyone

[+Cardinal Red]

Big_Lebowski Posted on Mar 18 2005, 03:50 PM

I have always noticed this unit has seemed to be off by about 1 football field round...

 

Just curious, what is your point of reference that indicates this level of innacuracy?

 

Often someone comes here and says " I was X number of feet too far away from the Cache". Or, "according to the Map I'm driving in the middle of the River".

 

Being a little more specific might help someone to answer your question.

[peter]

It would probably be cheaper to buy a basic used or even new GPS receiver than to get the auxiliary DGPS radio to connect to your GlobalNav.

 

How have you checked the level of accuracy? I'd start by getting a good lock with at least 5-6 satellites and mark a location somewhere near your home. The GlobalNav has a patch antenna in the top part of the case so hold the unit close to horizontal and a little away from your body so you don't block too much of the sky. Then a little later use the unit to "find" the same spot (again only after getting a good satellite lock) and see how close you are. Repeat a few times so you get a good feel for the amount of scatter in the measurements.

[+Big_Lebowski]

Big_Lebowski Posted on Mar 18 2005, 03:50 PM

I have always noticed this unit has seemed to be off by about 1 football field round...

 

Just curious, what is your point of reference that indicates this level of innacuracy?

 

Often someone comes here and says " I was X number of feet too far away from the Cache". Or, "according to the Map I'm driving in the middle of the River".

 

Being a little more specific might help someone to answer your question.

By saying aprox 1 football field round I get this measurment from

using Maptec online, and know EXACTLY where I want to go.

I went there, and when my GPS read what I pulled from their

server, I could see the point, but about a football field away.

 

I did make about 3 points of referance, and it seemd off about 1

football field round on each one.

 

im starting to think its the age of this unit.

and the restrictions in place at time of manufacturing

[vagabond]

The manual is available at the lowrance website

it is possible that its not set the same way the maps are

Edited March 18, 2005 by vagabond

[Hoary]

It sounds like wrong Datum?

 

And another option. This unit has:

"PCF (Position Correction Factor)

Another method used to make your display match a chart or map is called

“PCF” or Position Correction Factor. This unit gives you the capability to

move or offset the position shown on the display to match one on the

chart. The unit will add this offset to all position and navigation displays at

all times." (manual, p.41)

 

Overall this unit is similar to GlobalMap 100 (without maps, in Garmin analogy eTrex yellow and Legend) and should not be that inaccurate.

[+Cardinal Red]

Big_Lebowski Posted on Mar 18 2005, 04:30 PM

using Maptec online, and know EXACTLY where I want to go.

 

Just checked out Maptec online. I have very good coordinates on a Building about 2 Football fields long and 1 Football Field Wide. I used a Garmin Mapping unit set to WGS84 to obtain those coords.

 

Checking my known coords against that Building on Maptec, I found the Longitude to be quite close. But, the Latitude seems to be off by about 75 to 100 Feet (The pointer on the Map is sitting too far North).

 

So part of the problem could be the Maptec information itself.

[Neo_Geo]

That's a pretty HUGE error. One possibility is the confusion of the various Latitude/Longitude position formats. Look carefully at the differences between the following:

 

hddd.ddddd° = this is decimal degrees

hddd° mm.mmm' = this is degrees with decimal minutes

hddd° mm' ss.s" = this is degrees minutes and seconds

 

Geocaching uses the degrees with decimal minutes

 

Using this format, the White House is located at:

N38° 53.813' W77° 02.188'

 

Now, if I take the same numbers of those coordinates and plug them into degrees minutes and seconds format like this:

N38° 53' 81.3" W77° 02' 18.8"

 

18.8 seconds converts to about 0.313 minutes. The new waypoint is 6/10 of a mile to the NNW of the White House. That's actually about 10 football fields! The amount of this error can vary widely depending on the actual coordinates being used.

 

N38° 53.813' W77° 02.588' and N38° 53' 81.3" W77° 02' 58.8" are 9/10 of a mile apart.

 

Another thing to watch out for is the map datum setting. That should be set for WGS84. That would account for a smaller error - somewhat less than a football field.

[+IVxIV]

Does it seem you're always off your coordinates in the same direction? Meaning for example, is your target always 1-2 football fields away "southeast" of your current spot?

 

Sounds like a datum thing.. and don't waste your time getting DGPS to work. It's intention is to make a GPS even MORE accurate than it is by itself. And your unit is currently not accurate at all..

[+Big_Lebowski]

WOW!. Thank you for so many replies!!

 

On the Datums, I dont fully understand them

is there a specific one I should be using as of now?

I think im using what ever this unit defaulted to..

I may have changed it by playing with it back when.

 

I dont know as of now if its off by the same amount or

not or the same direction...

 

Mabey if there is a member around with some spare

time some time soon we can meet up somewhere

and compaire their reading to mine, mabey im reading

wrong and or maptec isnt showing me quite the same...

 

i have a copy of the manual printed and stuck in a 3 ring

binder..

 

anyone willing to meet up and see if the readings are close?

public place not a problem, Im by many major public places

Block of orange, Main place mall. etc etc etc

 

Thanks everyone!

great to see such a wide community with so many active and

helpfull members

[Neo_Geo]

...the map datum setting.  That should be set for WGS84.

Edited March 19, 2005 by Neo_Geo

[+WeightMan]

You might also try finding a benchmark. Those will quite often have the coords marked on the brass disk. The GC.com listing will have the coords in WGS84 even if the benchmark itself has the NAD27 version. That eliminates any problem with the map.

[+Cardinal Red]

Big_Lebowski Posted on Mar 19 2005, 02:25 AM

anyone willing to meet up and see if the readings are close?

I have no doubt that someone close to your location would welcome the opportunity to help you with this in person.

 

But, you have given us no clue that I can find, what your general location is.

 

Don't disclose in "public" exactly where you are, but give us a hint.

 

EDIT: Was this the "clue" I missed: Block of orange, Main place mall. etc etc etc

 

If it is, it certainly went right over my head.

 

Also there are Forums here by Country and by Region. Posting this in a vicinity specific Forum will give you the best chance to find the help you seek.

Edited March 19, 2005 by Cardinal Red

[+IVxIV]

Try using Maporama Enter your home address, and it will tell you your home coordiates.

 

Compare that with what the GPSr is showing,, it should be close.

[+Big_Lebowski]

Big_Lebowski Posted on Mar 19 2005, 02:25 AM

anyone willing to meet up and see if the readings are close?

I have no doubt that someone close to your location would welcome the opportunity to help you with this in person.

 

But, you have given us no clue that I can find, what your general location is.

 

Don't disclose in "public" exactly where you are, but give us a hint.

 

EDIT: Was this the "clue" I missed: Block of orange, Main place mall. etc etc etc

 

If it is, it certainly went right over my head.

 

Also there are Forums here by Country and by Region. Posting this in a vicinity specific Forum will give you the best chance to find the help you seek.

Anahiem angels stadium

[+Big_Lebowski]

...the map datum setting.  That should be set for WGS84.

Ok, found that as "default"

now reselected it to verify its the one in use..

 

Thanks =-)

 

[+Big_Lebowski]

Try using Maporama  Enter your home address, and it will tell you your home coordiates.

 

Compare that with what the GPSr is showing,, it should be close.

Hurmm, It seems close.

 

Let me post what i found and you guys tell me how off or inaccurate It sounds like?

 

Thanks everyone!!

 

Lat-Long:

match° match' 58" , -match° match' 13"

Mine shows .979' .214

 

Now there is different options to show position..

 

but these seem to be the closest match..

 

thank you all for your patence..

and help!!

Edited March 20, 2005 by Big_Lebowski

[+Big_Lebowski]

You might also try finding a benchmark. Those will quite often have the coords marked on the brass disk. The GC.com listing will have the coords in WGS84 even if the benchmark itself has the NAD27 version. That eliminates any problem with the map.

Thanks, I will look into these as well.

thanks for your patience and help

I will have a good knowledge base soon

[+MrCOgeo]

Lat-Long: 

match° match' 58" , -match° match' 13"

Mine shows    .979' .214

Could be the formats you are trying to compare. Both might be showing the aprox. same place just different formats in degrees, minutes and seconds vs. degrees minutes with decimal.

 

###° ##' ##" (or DDD° MM' SS")

 

is different format from common GC.com coord format:

 

###° ##.###' (or DDD° MM.MMM')

 

in other words...

38° 57' 12" -104° 41' 26"

 

38° 57.200' -104° 41.433'

 

are the same exact place but different displayed format.

 

Just trying to back up another helpful suggestion from Neo_Geo above

Edited March 20, 2005 by MRCOGEO

[+Big_Lebowski]

OK I HAVE NOW WENT OUT AND FOUND MY FIRST BENCH MARK!!!!

 

ANYONE WANNA TAKE A LOOK AT TELL ME APROX HOW OFF MY GPS IS LOOKING?

 

IMAGES TAKEN OF THE MARK AND GPS INCLUDED!!

 

WOOOOOOOHOOOOOOOOOOO!!!

 

 

ok sorry yelling a bit there

 

but if someone could tell me aprox what the Marks readings and

my gps as seen in the pictures what it looks like the difference

in feet aprox you would say that is?

 

http://www.geocaching.com/mark/details.aspx?PID=DX2223

 

I aprecate it i still dont know how to make seconds tenths of seconds into

feet yet! hehehe

soon im sure i will

 

thanks everyone!!

Edited March 20, 2005 by Big_Lebowski

[+Cardinal Red]

Big_Lebowski Posted on Mar 20 2005, 06:11 PM

if someone could tell me aprox what the Marks readings and my gps as seen in the pictures what it looks like the difference in feet aprox you would say that is?

Congratulations on this find ! ! !

 

Your GPS is set to the Geocaching standard of Degrees, Minutes, and Decimal Minutes.

 

The numbers in the "N" (North) row represent Latitude, and each .001 Minute equals about 6 Feet (anywhere on Earth).

 

The numbers in the "W" (West) row represent Longitude, and each .001 Minute equals about 5 Feet (at that particular location).

 

The Benchmark page gives N 33 Degrees 47.283 Minutes and W 117 Degrees 51.150 Minutes in the NAD83 Datum.

 

I know that we told you to use the WGS84 Datum, but NAD83 is virtually identical. So no problem there.

 

So: [295 - 283 = 12] ... [12 x 6 = 72 Feet] ... You are indicating 72 Feet too far North

 

And: [158 - 150 = 8] ... [8 x 5 = 40 Feet] ... You are indicating 40 Feet too far West

 

This is a total difference of about 82 Feet. I don't know how good a lock your GPSr had, but this is a lot better than a Football Field. I would certainly try a nearby Cache and see what happens.

 

Edited to add that if anyone else wants to make sense of this thread, I looked at the picture of your GPSr at the Benchmark.

It was indicating N 33 Degrees 47.295 Minutes and W 117 Degrees 51.158 Minutes.

Edited March 21, 2005 by Cardinal Red

[+Poindexter]

You need to find a benchmark with coordinates of a higher degree of accuracy. From the datasheet for this benchmark:

 

The horizontal coordinates were scaled from a topographic map and have

an estimated accuracy of +/- 6 seconds.

 

One second of latitude is approx 100' longitude will be somewhat less depending on your latitude.

[+Big_Lebowski]

Big_Lebowski Posted on Mar 20 2005, 06:11 PM

if someone could tell me aprox what the Marks readings and my gps as seen in the pictures what it looks like the difference in feet aprox you would say that is?

Congratulations on this find ! ! !

 

Your GPS is set to the Geocaching standard of Degrees, Minutes, and Decimal Minutes.

 

The numbers in the "N" (North) row represent Latitude, and each .001 Minute equals about 6 Feet (anywhere on Earth).

 

The numbers in the "W" (West) row represent Longitude, and each .001 Minute equals about 5 Feet (at that particular location).

 

The Benchmark page gives N 33 Degrees 47.283 Minutes and W 117 Degrees 51.150 Minutes in the NAD83 Datum.

 

I know that we told you to use the WGS84 Datum, but NAD83 is virtually identical. So no problem there.

 

So: [295 - 283 = 12] ... [12 x 6 = 72 Feet] ... You are indicating 72 Feet too far North

 

And: [158 - 150 = 8] ... [8 x 5 = 40 Feet] ... You are indicating 40 Feet too far West

 

This is a total difference of about 82 Feet. I don't know how good a lock your GPSr had, but this is a lot better than a Football Field. I would certainly try a nearby Cache and see what happens.

 

Edited to add that if anyone else wants to make sense of this thread, I looked at the picture of your GPSr at the Benchmark.

It was indicating N 33 Degrees 47.295 Minutes and W 117 Degrees 51.158 Minutes.

did i SaY 83? whoops sorry im on datum 84..

this thing has a list of a ton of them,.

Using datum84 tho

=-)

 

hurmm, 82 feet is pretty hefty,

then again i bought a old gps.

so mabey this error is in the software

and not as current as the newer when it

comes to accuracy?

 

But thanks for the reply a ton..

this helps a lot., and I think in next few days I will try and

find a cache..

thurs and fri I will have some spare time

 

Edited March 21, 2005 by Big_Lebowski

[+Cardinal Red]

Poindexter Posted on Mar 20 2005, 08:24 PM

You need to find a benchmark with coordinates of a higher degree of accuracy.

I was a little concerned about the accuracy of the Benchmark Coordinates also. But when I read the logs, a previous finder had provided coordinates of N 33 Deg. 47.280 Min. and W 117 Deg. 51.150 Min. So I was satisfied with using the very close numbers on the GC Benchmark Page.

[vagabond]

You might want to try a benchmark thats out in the open with no buildings around, as that one seems to be on a wall of a bank building

[+Poindexter]

But when I read the logs, a previous finder had provided coordinates of N 33 Deg. 47.280 Min. and W 117 Deg. 51.150 Min. So I was satisfied with using the very close numbers on the GC Benchmark Page.

Yes, that is only about 24' difference than the datasheet coordinates.

[vagabond]

I'm planning on caching in the area Easter Sunday, Probably starting in Orange. My wife will probably tag along, so all the caches will be easy ones. If you want to join us your welcome to come along. email me if your interested.

[gm100guy]

Try this test.

 

Get the topo map for the area where you live. Set your gps to give you the cords in utm. Turn on your gps and get a good sat lock on the first screen

 

Record a waypoint of a object that you can see on the map and when doing this let it average for about 2 minutes. Save that way point.

 

Now get your topo map that has the utm grid printed on it. Now you will need a roamer and match the scale to your map. Plot the utm postion from the gps onto the map.

 

You can then see what the gps postion is compared to what you should be able to read off the map.

 

This gps is 12 channel and is similar to to 2 gps's that I own and I don't have that much problem with the cords being that far out.

 

[+EScout]

As was mentioned earlier, this benchmark's location is scaled. You need to find one that is GPS "adjusted". A scaled mark can be hundreds of feet off. And an adjusted mark will be in an open area away from buildings.

 

NGS Benchmarks

 

Search by state, county, choose GPS marks only, and sort by lat or long to find one near you. Choose a disk, not a CORS. Due to rounding, the waypoint you enter into your GPSr is within about 3 feet of the mark.

 

This will give you a good idea of how well your GPSr works. I always find that I get within the EPE from the mark/waypoint.

[+Big_Lebowski]

I'm planning on caching in the area Easter Sunday, Probably starting in Orange. My wife will probably tag along, so all the caches will be easy ones. If you want to join us your welcome to come along. email me if your interested.

I will have to see what im doing, thats one heck of a offer tho, I may if in town just join up with you =-)

 

Would be nice to see what our GPS units show in comarison too

 

thanks

 

[+Big_Lebowski]

As was mentioned earlier, this benchmark's location is scaled.  You need to find one that is GPS "adjusted".  A scaled mark can be hundreds of feet off.  And an adjusted mark will be in an open area away from buildings.

 

NGS Benchmarks

 

Search by state, county, choose GPS marks only, and sort by lat or long to find one near you.  Choose a disk, not a CORS.  Due to rounding, the waypoint you enter into your GPSr is within about 3 feet of the mark.

 

This will give you a good idea of how well your GPSr works.  I always find that I get within the EPE from the mark/waypoint.

Looking into thoes now..

holy snot it lists a ton.

 

I aprecate you all putting up with my

Dingy newness

Edited March 21, 2005 by Big_Lebowski

[+EScout]

One additional bit of info:

The coords for are listed in degrees, minutes and seconds, decimal seconds (ddd mm ss.sssss), so convert this to degrees, minutes, decimal minutes (ddd mm.mmm).

 

For GPS adjusted marks, they are listed to the one hundred thousandth of a second.

[+Cardinal Red]

EScout Posted on Mar 20 2005, 11:26 PM

As was mentioned earlier, this benchmark's location is scaled.  You need to find one that is GPS "adjusted".  A scaled mark can be hundreds of feet off.

 

Normally I would agree with you, but as was also mentioned earlier:

 

Cardinal Red Posted on Mar 20 2005, 08:41 PM

I was a little concerned about the accuracy of the Benchmark Coordinates also. But when I read the logs, a previous finder had provided coordinates of N 33 Deg. 47.280 Min. and W 117 Deg. 51.150 Min. So I was satisfied with using the very close numbers on the GC Benchmark Page.

 

Poindexter Posted on Mar 20 2005, 09:07 PM

Yes, that is only about 24' difference than the datasheet coordinates.

That is actually only about 18' different than the GC Coords. Certainly not hundreds of feet.

 

In this particular case I would be more concerned about:

 

vagabond Posted on Mar 20 2005, 09:05 PM

You might want to try a benchmark thats out in the open with no buildings around, as that one seems to be on a wall of a bank building

[+AtoZ]

By th fiirmware version 1998 it is pre SA being turned off. I think you unit may only have two decimal units and it may use sequential satillites instead of parallel aquisition. It seems it is an older unit and there for they ahd accuraceies of like 200 feet. That is just what I can assume without looking at the specs for the unit.

Maybe time to buy a Etrex.

cheers

[vagabond]

Also you might want to try socalgeocachers.com there are quite a few cachers in your area that use it and I'm sure one or more of them would be glad to help you.

Edited March 22, 2005 by vagabond

[+Cardinal Red]

AtoZ Posted on Mar 22 2005, 10:36 AM

I think you unit may only have two decimal units

He gave us a 3 Decimal picture (at the Benchmark) in his Log

[peter]

By th fiirmware version 1998 it is pre SA being turned off. I think you unit may only have two decimal units and it may use sequential satillites instead of parallel aquisition. It seems it is an older unit and there for they ahd accuraceies of like 200 feet.

The specifications of units at that time (when SA was active) frequently just cited the SA numbers; i.e. 300' of horizontal accuracy. But the Lowrance Global12 had 12 parallel channels (hence the '12' in the name) and gives just as many decimal places as today's consumer models. I'd expect it to give about the same accuracy as any modern unit with the exception that it won't support WAAS. Also the Lowrance units of that time didn't correct for the geoid vs. ellipsoid altitudes, so they will show a systematic elevation error compared to mean sealevel. But that doesn't affect the horizontal lat/long positions.

[n7yaf]

I have the same unit, have had it for about 3 years. (Replaced an earlier one that I dropped.) It has an accuracy of about 6 feet on most of the geocaches I have worked. I am using the wgs84, and no offset adjustment.

It's memory died and when the regular batteries were replaced all information was lost. Lowrance gave absolutely help except recommend buy a new one. Finally got mad and took it apart. It has a $3.00 battery for memory backup. Replaced it and now memory is fine.

 

Good luck on the unit, you may have to work the offset adjustment.

Allen

[+EScout]

n7yaf:

Good job on replacing the internal memory battery. If you keep an electronic device long enough, the internal battery will need replacing.

 

73, GL

Steve

[+Big_Lebowski]

I'm planning on caching in the area Easter Sunday, Probably starting in Orange. My wife will probably tag along, so all the caches will be easy ones. If you want to join us your welcome to come along. email me if your interested.

 

My fiencee didnt tell me untill yesterday she wants to spend

Easter with in laws.. Sooooooo I will have to miss this chance

to meet up unfortunaltey.

.

My work week is screwy right now too.

I work mon, tue, wends, sat,

 

Mabey we can meet up another time and hunt some down

[+Big_Lebowski]

I have the same unit, have had it for about 3 years.  (Replaced an earlier one that I dropped.)    It has an accuracy of about 6 feet on most of the geocaches I have worked.  I am using the wgs84, and no offset adjustment.

  It's memory died and when the regular batteries were replaced all information was lost.  Lowrance gave absolutely help except recommend buy a new one.  Finally got mad and took it apart.  It has a $3.00 battery for memory backup.  Replaced it and now memory is fine. 

 

Good luck on the unit, you may have to work the offset adjustment.

Allen

Hey Tad, If you are ever in the so-cal area,

gimme a shout..

really would like to compaire the units side by side,

check out software versions, and see what the status

windows show happening different on the screens...

 

(message edit to add in)

ALSO DID YOU GET THE COMM CABLE>!?!

you know where i can get one and mabey a replacement

cover for the connector??)

 

thanks

 

talk to ya soon

Edited March 24, 2005 by Big_Lebowski

[hgmonaro]

  It's memory died and when the regular batteries were replaced all information was lost.  Lowrance gave absolutely help except recommend buy a new one.  Finally got mad and took it apart.  It has a $3.00 battery for memory backup.  Replaced it and now memory is fine. 

Allen, what were the symptoms? I have a GlobalNav and it shuts down straight away after turning on. Like you, I got no joy from Lowrance (the fix cost was a set price regardless of problem and a good portion of the price of a new one) and have never done anything about it. I made one half-attempt at pulling it apart but felt like I was going to break the clamshell trying to snap it open (after removing screws inside battery compartment) Can you remember where the locking points were?

 

For the record and original poster, when it worked it was accurate.

 

Regards, Nige

[hgmonaro]

Just adding a follow up in case someone else has a similar problem and reads this thread...

 

Got email from Allen describing his units symptons and although not identical, they were similar. I thought I'd pulled it (mines a Globalnav 200 BTW) apart but must have been thinking of something else cause I hadn't. Didn't have a suitable screwdriver which you need. A 'Torx' #7. Got one of them today and a battery (CR2450) and replaced it. It fixed it! At the moment I'v just got the power wires taped to the battery because they were connected to spot-welded tabs on the original. I'll have to have a go at soldering them on although from past experience trying to do that with car key fobs, it ain't easy to solder to a battery (and probably dangerous too!)

 

Anyway... my Globalnav is back working and I can resume caching after a big break!