Garmin GPS - How to determine GPS accuracy?

Author

How to determine GPS accuracy?

Father Guido

2008-02-12, 4:33 am

Hi,

I got a Nuvi 250 for Christmas, and that was my first GPS. I'm curious
about it's accuracy. I'm also in Canada, so finding pre-determined
locations to check against isn't possible (I don't think). The arrow
seems to match my location on street maps, but I'd like to verify
accuracy as a measure of +/- 20', or +/- 30' etc. Any ideas?

Thanks,

Norm

zydecogary

2008-02-12, 4:33 am

Norm

This is not the final answer -- but you can easily
experiment until you determine where the nearest 'real'
geodetic marker is and use that to compare with your Garmin.

Anyway using http://maps.google.com or Google Earth, find
a spot near your house where you can zoom in and really
find a small spot that you will actually be able to get to -
- and read the latitude and longitude.

For getting coordinates from Google see:
Garmin GPS - Tricks, Tips, Work Arounds, Hints, Secrets
and Ideas
Article:

ARE YOU COORDINATED? -
Obtaining and Using Coordinates in Your GPS

Now, go to that spot (with your antenna up/or in active
GPS mode) and read from your instrument what your Garmin is
reflecting as the coordinates.
See Garmin GPS Tricks, Tips, etc.........
Article:

WHERE AM I? BY THE NUMBERS -
Coordinates of Your PRESENT Position

Make a comparison of the coordinates.

You might walk around the spot, watching the coordinates
change until they match what Google maps said (assuming
Google was the most accurate of coordinate indicators in
the world) and eye ball or measure the straight line
difference.

You could even plan a route by making the Google
coordinates a destination and your reality position as the
starting point and let your GPS tell you how many feet your
are away from your destination. You would read the distance
from a 'Where to?' inner screen. [I didn't try this but it
should work -- somewhat.]

I hope that some of this is some help. Eh?

Read more about
Garmin GPS Tricks, Tips, Work Arounds, Secrets and Hints at

********************
**
http://snipurl.com/garmin_gps_tricks
********************
**

Gary Hayman

=3D=3D=3D=3D=3D=3D=3
D=3D=3D=3D=3D=3D=3D=
3D=3D=3D=3D=3D=3D=3D
=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3
D=3D=3D=3D=3D=3D=3D=
3D=3D=3D

On Feb 12, 2:36=A0am, Father Guido <F...@no.where> wrote:
> Hi,
>
> I got a Nuvi 250 for Christmas, and that was my first GPS. I'm curious
> about it's accuracy. I'm also in Canada, so finding pre-determined
> locations to check against isn't possible (I don't think). The arrow
> seems to match my location on street maps, but I'd like to verify
> accuracy as a measure of +/- 20', or +/- 30' etc. Any ideas?
>
> Thanks,
>
> Norm

keith

2008-02-12, 4:33 am

On Tue, 12 Feb 2008 07:36:39 +0000, Father Guido wrote:

> Hi,
>
> I got a Nuvi 250 for Christmas, and that was my first GPS. I'm curious
> about it's accuracy. I'm also in Canada, so finding pre-determined
> locations to check against isn't possible (I don't think). The arrow
> seems to match my location on street maps, but I'd like to verify
> accuracy as a measure of +/- 20', or +/- 30' etc. Any ideas?
>
> Thanks,
>
> Norm

Accuracy depends on how many satellites have been locked. On the 300
series, the 'where to' screen shows signal strength bars in the top left
hand corner. Tap these an you go to a screen which indicates which
signals have been acquired, and what the expected accuracy is likely to
be. I suspect the 250 is similar.

Keith

Dale Atkin

2008-02-12, 12:33 pm

> I'm also in Canada, so finding pre- determined
> locations to check against isn't possible (I don't think).

There are many places you can get pre-surveyed locations from online, even
in Canada.

Here is one option
http://www.waymarking.com/cat/detai...235c49&exp=True

Don't forget to turn off the lock on road 'feature' before checking
accuracy.

Dale

Father Guido

2008-02-12, 10:33 pm

On Tue, 12 Feb 2008 07:36:39 GMT, Father Guido <FG@no.where> wrote:

>Hi,
>
>I got a Nuvi 250 for Christmas, and that was my first GPS. I'm curious
>about it's accuracy. I'm also in Canada, so finding pre-determined
>locations to check against isn't possible (I don't think). The arrow
>seems to match my location on street maps, but I'd like to verify
>accuracy as a measure of +/- 20', or +/- 30' etc. Any ideas?
>
>Thanks,
>
>Norm

Thanks to everyone who replied, I have a few methods to try out, and
it'll be good practice in using some features I haven't used before.

Thanks again,

Norm

James Robinson

2008-02-13, 4:33 am

zydecogary <zydecogary@gmail.com> wrote:

> Norm
>
> This is not the final answer -- but you can easily
> experiment until you determine where the nearest 'real'
> geodetic marker is and use that to compare with your Garmin.
>
> Anyway using http://maps.google.com or Google Earth, find
> a spot near your house where you can zoom in and really
> find a small spot that you will actually be able to get to -
> - and read the latitude and longitude.

Don't bet on the accuracy of Google Earth. The mapping is approximate at
best.

Bruce in alaska

2008-02-13, 3:33 pm

In article < Xns9A42F342EAA18wasc
ana212com@66.250.146.128>,
James Robinson <wascana@212.com> wrote:

> zydecogary <zydecogary@gmail.com> wrote:
>
>
> Don't bet on the accuracy of Google Earth. The mapping is approximate at
> best.

Google does pretty good on their accuracy. Last year I had "The Google
Guy" charter a Floatplane, and fly out to my, very isolated Inlet,
here in alaska, to take Survey Grade GPS Fixes, of specific geographic
locations, that could be seen, and Identified, on upgraded SAT based
Photos. He said that the updated Registrations would be available in
about 6 months. Sure enough, about six months later, the High Resolution
Pictures were incorporated, and the GPS Registrations were "Right On"
One of the Points, he Registered, was a USGS Monument that was Survey'd
in by the USGS a few years back, as a Primary Survey Monument in our
area.

--
Bruce in alaska
add <path> after <fast> to reply

James Robinson

2008-02-13, 3:33 pm

Bruce in alaska <fast@btpost.net> wrote:

> In article < Xns9A42F342EAA18wasc
ana212com@66.250.146.128>,
> James Robinson <wascana@212.com> wrote:
>
>
> Google does pretty good on their accuracy.

While they aren't too bad in general, if you look closely at the photos,
you will often see mismatches at the mosaic boundaries. Roads can jog
perhaps 50 or 100 feet at times. Those jogs can only mean there are
errors. I have also seen political boundaries several hundred feet from
where they should be.

If someone was trying to look at the accuracy of their GPS, they would
have to recognize that those sorts of errors can exist in the Google
data.

Bruce

2008-02-13, 10:33 pm

"James Robinson" <wascana@212.com> wrote in message
news:Xns9A43A0E437FA
3wascana212com@66.250.146.128...
> Bruce in alaska <fast@btpost.net> wrote:
>
>
> While they aren't too bad in general, if you look closely at the photos,
> you will often see mismatches at the mosaic boundaries. Roads can jog
> perhaps 50 or 100 feet at times. Those jogs can only mean there are
> errors. I have also seen political boundaries several hundred feet from
> where they should be.
>
> If someone was trying to look at the accuracy of their GPS, they would
> have to recognize that those sorts of errors can exist in the Google
> data.

Just for fun I compared my location from Google Earth and my Garmin
GPS60CSx.
I am in the Washington DC area. According to this applet:
http://www.movable-type.co.uk/scrip...g-vincenty.html the difference
was 7.6
meters, or about 21 feet. Although it's hard to tell I think my location is
near the
center of a map. At any rate I was somewhat surprised at how close they
were,
although I don't think you could draw any conclusions from that except that
maybe
the correleation between Google Earth and GPS is something like 21 feet in
some circumstances.

Happy Trails

2008-02-14, 4:33 am

On Wed, 13 Feb 2008 19:22:29 -0500, "Bruce" <null@null.com> wrote:

>I was somewhat surprised at how close they
>were,
>although I don't think you could draw any conclusions from that except that
>maybe
>the correleation between Google Earth and GPS is something like 21 feet in
>some circumstances.
>

I have found GE to be accurate enough to use without further
calibration for drawing roads on islands, towns and cities in S.
Philippines, in locations where hi-res images are available. In some
cases, upon casual verification (no long time data collection) I have
seen it off by as much as 30 meters maximum, but that's in a "spend an
afternoon - make a map" quickie project. For anything in the
countryside this is fine, as there are not two parallel roads to get
mixed up. It's nice because it can take a major, time-consuming and
somewhat expensive chore right out of the map-making process.

In the cities I actually have to get off my XXX and go outside and
have to take a couple of readings to shift the map a bit to match GE
perfectly. You will notice that the roads turned on in GE do not
usually match their image perfectly. Usually, when navigating, I have
the zoom level set such that you can hardly see even 30 meters
difference between position and road anyway, so it becomes irrelavent.

Topaz305RK

2008-02-14, 10:33 am

To true.
In our section of Montana, Google Earth has the American/Canadian border
approx. 800 yards south of the real line.
Nice yellow line on Google Earth but you can see from the farmers fields
where "The Line" really is.
Also from the location of the "Ports" on "The Line".

Rupert Brooke

2008-02-14, 10:33 am

Doesn't Google Earth have better accuracy for it's PAID membership, than for
the Free area many of us use?

It is my understanding that Garmin was instrumental with the establishing
work of the GPS system with the military. So they should have accuracy
capability. (Garmin gets patent royalties from other GPS makers.) When GPS
first came out, there accuracy was about 300 feet for the public, because
the military thought it should be that way. President Clinton did away with
that and said we were allowed better. But it still isn't on par with US
military accuracy, so terrorists can't use it...

Hope this doesn't sound too 'conspiracy theorist'!
Rupert

"Bruce" <null@null.com> wrote in message
news:hRLsj.42958$Ft5.14097@newsfe15.lga...
>
> "James Robinson" <wascana@212.com> wrote in message
> news:Xns9A43A0E437FA
3wascana212com@66.250.146.128...
>
> Just for fun I compared my location from Google Earth and my Garmin
> GPS60CSx.
> I am in the Washington DC area. According to this applet:
> http://www.movable-type.co.uk/scrip...g-vincenty.html the difference
> was 7.6
> meters, or about 21 feet. Although it's hard to tell I think my location
> is near the
> center of a map. At any rate I was somewhat surprised at how close they
> were,
> although I don't think you could draw any conclusions from that except
> that maybe
> the correleation between Google Earth and GPS is something like 21 feet in
> some circumstances.
>

Happy Trails

2008-02-14, 10:33 am

On Thu, 14 Feb 2008 06:47:44 -0700, "Topaz305RK" <k7sam@arrl.net>
wrote:

>In our section of Montana, Google Earth has the American/Canadian border
>approx. 800 yards south of the real line.
>Nice yellow line on Google Earth but you can see from the farmers fields
>where "The Line" really is.
>Also from the location of the "Ports" on "The Line".
>

Duh!

When we are discussing GE accuracy, we are talking about how close the
coordinate display of a recognizable point in an image is to its
actual coordinates - NOT where they chose to overlay their
representative imaginary lines for streets and borders and such!

James Robinson

2008-02-14, 10:33 pm

James Robinson <wascana@212.com> wrote:

> zydecogary <zydecogary@gmail.com> wrote:
>
>
> Don't bet on the accuracy of Google Earth. The mapping is approximate
> at best.

Here's an example I ran across of how they stitch photos together. You can
clearly see the mismatch:

http://tinyurl.com/2u8nvt

Topaz305RK

2008-02-14, 10:33 pm

"Happy Trails" <nomail@myplace.com> wrote in message
news:tf6ar3dsamhqbjd
nfo5nkgj8gncu4ecuja@
4ax.com...
> On Thu, 14 Feb 2008 06:47:44 -0700, "Topaz305RK" <k7sam@arrl.net>
> wrote:
>
>
> Duh!
>
> When we are discussing GE accuracy, we are talking about how close the
> coordinate display of a recognizable point in an image is to its
> actual coordinates - NOT where they chose to overlay their
> representative imaginary lines for streets and borders and such!

Yea...........and..................
So we should all ASSUME that if they can't put the United States and
Canadian border line in the correct position that everything else is just
fine and dandy - is that what you are saying - HELLO - Wake up and smell the
coffee. If you find a number of inconsistencies then how do you decide what
is correct and what is incorrect?

Happy Trails

2008-02-15, 7:34 am

On Thu, 14 Feb 2008 21:13:57 -0700, "Topaz305RK" <k7sam@arrl.net>
wrote:

>Yea...........and..................
>So we should all ASSUME that if they can't put the United States and
>Canadian border line in the correct position that everything else is just
>fine and dandy - is that what you are saying - HELLO - Wake up and smell the
>coffee. If you find a number of inconsistencies then how do you decide what
>is correct and what is incorrect?
>

I get off my XXX and go outside with a gps receiver and check it. In
many cases those lines are taken off old maps, and do not have much
chance of being accurate, but the imaging - that's where someone has
put some actual effort into calibrating. I care about the images - I
don't care about the lines. I usually have them turned off anyway.

If you want to believe that GE is no good because they really think
the border is that far out, please yourself.

Jack Erbes

2008-02-15, 10:34 am

Father Guido wrote:
> Hi,
>
> I got a Nuvi 250 for Christmas, and that was my first GPS. I'm curious
> about it's accuracy. I'm also in Canada, so finding pre-determined
> locations to check against isn't possible (I don't think). The arrow
> seems to match my location on street maps, but I'd like to verify
> accuracy as a measure of +/- 20', or +/- 30' etc. Any ideas?
>

Does Canada have a counterpart agency for the U.S. National Geodetic
Survey office?

http://www.ngs.noaa.gov/

The NGS maintains a collection of datasheets for monuments that can be
real useful for comparing or evaluating GPS receiver performance. There
is a bit of a learning curve on getting them and using them but here is
the starting point for us:

http://www.ngs.noaa.gov/cgi-bin/datasheet.prl

Reading the history of some of the marks and the comments and
descriptions for finding the marks can be pretty entertaining.

If you are near any railroad right of ways, you can nearly always find
survey marks near them. The marks will often be mounted on or embedded
in nearby rock formations and are very permanent in nature.

Another source of public information that might help you are the
publicly available NOAA navigation charts:

http://nauticalcharts.noaa.gov/staff/charts.htm

Some of those cover the shared waters between the U.S. and Canada and
you should be able to find the locations of lighthouses and daymarks
express in very accurate terms on those charts.

The navigation details on the ENC or vector charts will have attributes
for each item seen and things like lighthouses will have their locations
expressed in to accuracies like 44 13.345N 68 20.222W (that is the Bass
Head Lighthouse). So there you have an object that is easily
approachable by land and on that the location of the red light on the
lighthouse tower is described to an accuracy of plus or minus one foot
or so.

http://nauticalcharts.noaa.gov/mcd/enc/index.htm

The RNC or raster charts are less useful as those do not have the
attributes for the objects (they are bitmapped images). But they will
have a specified location for some of the more prominent or important
objects seen

http://nauticalcharts.noaa.gov/mcd/Raster/Index.htm

Those pages also have links for viewers for looking at the charts with.

If you're really interested in studying accuracy at the "seat of the
pants" or consumer level, you might just have to figure out how your
national system works and where the data is.

Down here, the cost of accumulating and managing the data is paid with
our tax Dollars. As a result of that, the info is in the public domain.
We paid for it, we are entitled to it. It is that simple.

Some countries are not as generous about sharing information with
taxpayers and I think Canada might be in that category. That might
inhibit the process a little for you.

Jack

James Robinson

2008-02-15, 10:34 am

Jack Erbes < jackerbes@roadrunner
.com> wrote:
>
> Does Canada have a counterpart agency for the U.S. National Geodetic
> Survey office?
>
> http://www.ngs.noaa.gov/

National Resources Canada provides that service:

http://www.geod.nrcan.gc.ca/index_e.php

Jack Erbes

2008-02-15, 12:34 pm

James Robinson wrote:
> Jack Erbes < jackerbes@roadrunner
.com> wrote:
>
> National Resources Canada provides that service:
>
> http://www.geod.nrcan.gc.ca/index_e.php

I knew there had to be one, thanks for the link.

I really like the look of and content in the Canadian topo maps, I was
looking at some samples of the 1:50,000 topo maps here:

http://maps.nrcan.gc.ca/topo_e.php

I guess those are not available as electronic maps?

I'd love to see our USGS go to something like those for our maps. And
continue to make them available for free downloads of course. The
detail on those is good enough that if they want to those in a vector
format that would autoroute and with that amount of detail and content,
I could live with one kind of mapping for everything on my handheld.

Jack

James Robinson

2008-02-15, 12:34 pm

Jack Erbes < jackerbes@roadrunner
.com> wrote:
>
> James Robinson wrote:
>
> I knew there had to be one, thanks for the link.
>
> I really like the look of and content in the Canadian topo maps, I was
> looking at some samples of the 1:50,000 topo maps here:
>
> http://maps.nrcan.gc.ca/topo_e.php
>
> I guess those are not available as electronic maps?

The Canadian topo system is available electronically. Here is the web
site for topo mapping. The digital stuff is toward the bottom of the
page:

http://atlas.nrcan.gc.ca/site/engli...topo/index.html

A student has a web site where he has been converting the data for use in
Garmin GPS units:

http://www.ibycus.com/ibycustopo/

Father Guido

2008-02-19, 4:33 am

On Fri, 15 Feb 2008 09:17:56 -0500, Jack Erbes
< jackerbes@roadrunner
.com> wrote:

>Father Guido wrote:
>
>Does Canada have a counterpart agency for the U.S. National Geodetic
>Survey office?
>
>http://www.ngs.noaa.gov/
>
>The NGS maintains a collection of datasheets for monuments that can be
>real useful for comparing or evaluating GPS receiver performance. There
>is a bit of a learning curve on getting them and using them but here is
>the starting point for us:
>
>http://www.ngs.noaa.gov/cgi-bin/datasheet.prl
>
>Reading the history of some of the marks and the comments and
>descriptions for finding the marks can be pretty entertaining.
>
>If you are near any railroad right of ways, you can nearly always find
>survey marks near them. The marks will often be mounted on or embedded
>in nearby rock formations and are very permanent in nature.
>
>Another source of public information that might help you are the
>publicly available NOAA navigation charts:
>
>http://nauticalcharts.noaa.gov/staff/charts.htm
>
>Some of those cover the shared waters between the U.S. and Canada and
>you should be able to find the locations of lighthouses and daymarks
>express in very accurate terms on those charts.
>
>The navigation details on the ENC or vector charts will have attributes
>for each item seen and things like lighthouses will have their locations
>expressed in to accuracies like 44 13.345N 68 20.222W (that is the Bass
>Head Lighthouse). So there you have an object that is easily
>approachable by land and on that the location of the red light on the
>lighthouse tower is described to an accuracy of plus or minus one foot
>or so.
>
>http://nauticalcharts.noaa.gov/mcd/enc/index.htm
>
>The RNC or raster charts are less useful as those do not have the
>attributes for the objects (they are bitmapped images). But they will
>have a specified location for some of the more prominent or important
>objects seen
>
>http://nauticalcharts.noaa.gov/mcd/Raster/Index.htm
>
>Those pages also have links for viewers for looking at the charts with.
>
>If you're really interested in studying accuracy at the "seat of the
>pants" or consumer level, you might just have to figure out how your
>national system works and where the data is.
>
>Down here, the cost of accumulating and managing the data is paid with
>our tax Dollars. As a result of that, the info is in the public domain.
> We paid for it, we are entitled to it. It is that simple.
>
>Some countries are not as generous about sharing information with
>taxpayers and I think Canada might be in that category. That might
>inhibit the process a little for you.
>
>Jack

Thanks Jack,

You are spot on about Canada. They collect the data using our taxes
and then sell the information back to us.

It seems to me that I have seen a number of circular brass plates
imbedded in roadways throughout our city. I'll have to have a good
look at one and see what they say -- I'm sure they are survey markers
of some kind. Thanks again for your info!

Norm

Jack Erbes

2008-02-19, 10:33 pm

Father Guido wrote:
> On Fri, 15 Feb 2008 09:17:56 -0500, Jack Erbes
> < jackerbes@roadrunner
.com> wrote:
>
>
> Thanks Jack,
>
> You are spot on about Canada. They collect the data using our taxes
> and then sell the information back to us.
>
> It seems to me that I have seen a number of circular brass plates
> imbedded in roadways throughout our city. I'll have to have a good
> look at one and see what they say -- I'm sure they are survey markers
> of some kind. Thanks again for your info

With the link that James Robinson posted:

http://www.geod.nrcan.gc.ca/index_e.php

and some research you may be able to find some online databases that
will give you more info about those plates if they are benchmarks.

I played around with finding some a little bit and trying to compare
fixes on my handheld to the monuments (the various pins, rods, brass
plates, church steeples, etc., as described). It was interesting but
generally inconclusive as far as coming up with any stated standards of
accuracy. It pretty much boiled down to that my GPS fix and the bench
mark were always pretty much in agreement and with the EPE as stated on
the GPS.

I found the effort and exercise in finding the marks to be the most
interesting part of it all. That and thinking back on what some of the
marks could sway if they could talk. Some of them have seen most of the
history of U.S. railroads, as much as two hundred years of passing
ships, etc.

Benchmarking and benchmark hunting is a hobby for some, you may be able
to find a group up there if you get seriously interested.

Jack

Father Guido

2008-02-20, 4:33 am

On Tue, 19 Feb 2008 17:03:54 -0500, Jack Erbes
< jackerbes@roadrunner
.com> wrote:

>Father Guido wrote:
>
>With the link that James Robinson posted:
>
>http://www.geod.nrcan.gc.ca/index_e.php
>
>and some research you may be able to find some online databases that
>will give you more info about those plates if they are benchmarks.
>
>I played around with finding some a little bit and trying to compare
>fixes on my handheld to the monuments (the various pins, rods, brass
>plates, church steeples, etc., as described). It was interesting but
>generally inconclusive as far as coming up with any stated standards of
>accuracy. It pretty much boiled down to that my GPS fix and the bench
>mark were always pretty much in agreement and with the EPE as stated on
>the GPS.
>
>I found the effort and exercise in finding the marks to be the most
>interesting part of it all. That and thinking back on what some of the
>marks could sway if they could talk. Some of them have seen most of the
>history of U.S. railroads, as much as two hundred years of passing
>ships, etc.
>
>Benchmarking and benchmark hunting is a hobby for some, you may be able
>to find a group up there if you get seriously interested.
>
>Jack

Excellent information, I appreciate it. Like you hinted there's
nothing I can do if it isn't accurate, it's more of an interest just
to see how close it is, like I said I can't change the accuracy, so
this is just for fun. I'll look for some of those plates this weekend
and see what they say. For sure these plaques haven't seen more than a
few years of history, rather than centuries. Thanks again. I'll update
the group on what I find, in case anyone cares.

Norm

Bruce in alaska

2008-02-20, 12:33 pm

In article < 47bb525d$0$16694$4c3
68faf@roadrunner.com>,
Jack Erbes < jackerbes@roadrunner
.com> wrote:

> With the link that James Robinson posted:
>
> http://www.geod.nrcan.gc.ca/index_e.php
>
> and some research you may be able to find some online databases that
> will give you more info about those plates if they are benchmarks.
>
> I played around with finding some a little bit and trying to compare
> fixes on my handheld to the monuments (the various pins, rods, brass
> plates, church steeples, etc., as described). It was interesting but
> generally inconclusive as far as coming up with any stated standards of
> accuracy. It pretty much boiled down to that my GPS fix and the bench
> mark were always pretty much in agreement and with the EPE as stated on
> the GPS.
>
> I found the effort and exercise in finding the marks to be the most
> interesting part of it all. That and thinking back on what some of the
> marks could sway if they could talk. Some of them have seen most of the
> history of U.S. railroads, as much as two hundred years of passing
> ships, etc.
>
> Benchmarking and benchmark hunting is a hobby for some, you may be able
> to find a group up there if you get seriously interested.
>
> Jack

Jack, The one thing I did, when there was a Surveyor working in my
Area, was to have him survey in a Point, next to my Office, and I
setup one of my Garmin GPS3+'s external antenna on that point. The
point was surveyed to 10 cm, and I now have a reference to use, for
Post-Processing of GPS Fixes and Tracks from my other GPS3+, when
I use it out in the field. This is good for anywhere within a couple
of hundred Square Miles. I just take the Reference Data, Find the
difference between that, and the Surveyed Point, then add the Correction
Factor to my Field Data with the same Date and TimeStamp. This gets me
sub Meter Resolution, albeit not in Realtime but still sub Meter
Resolution.

--
Bruce in alaska
add <path> after <fast> to reply

Jack Erbes

2008-02-20, 3:33 pm

Bruce in alaska wrote:
<snip>
> Jack, The one thing I did, when there was a Surveyor working in my
> Area, was to have him survey in a Point, next to my Office, and I
> setup one of my Garmin GPS3+'s external antenna on that point. The
> point was surveyed to 10 cm, and I now have a reference to use, for
> Post-Processing of GPS Fixes and Tracks from my other GPS3+, when
> I use it out in the field. This is good for anywhere within a couple
> of hundred Square Miles. I just take the Reference Data, Find the
> difference between that, and the Surveyed Point, then add the Correction
> Factor to my Field Data with the same Date and TimeStamp. This gets me
> sub Meter Resolution, albeit not in Realtime but still sub Meter
> Resolution.

That is all pretty much over my head as far as my knowledge and uses of
GPS. When you say "GPS3+" are you talking about these long
discontinued, consumer grade, "GPS III Plus" receivers?

https://buy.garmin.com/shop/shop.do?pID=121&tab=gpsiiip

And does that mean you can apply the error that existed at the reference
point to other fixes taken within 200 miles, and at the same time, and
get 10cm accuracy?

I'm a retired machinist of sorts, I've spent way too much of my life
agonizing over measurements and how many fairies can dance on the head
of a pin. I don't even want to get started on that with GPS. I like
"close enough", that's all I really want to know. :> )

Jack

Bruce in alaska

2008-02-21, 12:34 pm

In article < 47bc9616$0$24118$4c3
68faf@roadrunner.com>,
Jack Erbes < jackerbes@roadrunner
.com> wrote:

> Bruce in alaska wrote:
> <snip>
>
>
> That is all pretty much over my head as far as my knowledge and uses of
> GPS. When you say "GPS3+" are you talking about these long
> discontinued, consumer grade, "GPS III Plus" receivers?
>
> https://buy.garmin.com/shop/shop.do?pID=121&tab=gpsiiip
>
> And does that mean you can apply the error that existed at the reference
> point to other fixes taken within 200 miles, and at the same time, and
> get 10cm accuracy?
>
> I'm a retired machinist of sorts, I've spent way too much of my life
> agonizing over measurements and how many fairies can dance on the head
> of a pin. I don't even want to get started on that with GPS. I like
> "close enough", that's all I really want to know. :> )
>
> Jack

Yes, these are the OLD Garmin GPS3+ 12 Channel Receiver Units. Yes,
you can use Post-Processing to correct and or refine GPS fixes. If
your reference is a couple of "cm" you can easily get sub Meter
resolutions. Usually the corrected resolution is about three times the
resolution of the Reference. So say your Reference is 10cm, then your
corrected resolution is in the neighborhood of 30 to 40cm, and is valid
for fixes within about 150 miles from the Reference Point. This is the
same thing that DGPS does, in except they use the Old MF Beacon
Transmitters to broadcast the Corrections in Realtime. Surveyors use
this Post-Processing technique, to refine their Field Work, as they
usually subscribe to a Service that collects Correction Data in Realtime,
and then sells the Data to Surveyors, by Time, Date, and Area out of the
Database.

--
Bruce in alaska
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