My brother the civil engineer asked me (the supposed family GPS
expert) about the following scenario (to help in developing initial
site plans without immediately having to go to expensive centimeter-
accurate surveying GPSs and/or bringing in and paying real surveyors,
which would come later on in the process):

Take a pair of cheap identical datalogging GPSs. Park one at a
reference point and let it sit there logging. Send cheapo employee
around with the other unit to the points of interest in the site.
Gather the two and dump out their logs. Match the timestamps, check
that both units used the same satellite set at the same times
(probable over a limited area, like a large construction site?), and
calculate all the relative offsets between the points of interest and
the reference. How accurate would those offsets be? Especially
altitudes (important for things like sewer lines)?

I'm guessing pretty accurate, like down to the resolution of the
units. Say 6 inches for a GPS that reported minutes with 5 digits to
the right of the decimal point.

Thoughts?

--
Silvar Beitel

"Silvar Beitel" wrote in message
...
My brother the civil engineer asked me (the supposed family GPS
expert) about the following scenario (to help in developing initial
site plans without immediately having to go to expensive centimeter-
accurate surveying GPSs and/or bringing in and paying real surveyors,
which would come later on in the process):

Take a pair of cheap identical datalogging GPSs. Park one at a
reference point and let it sit there logging. Send cheapo employee
around with the other unit to the points of interest in the site.
Gather the two and dump out their logs. Match the timestamps, check
that both units used the same satellite set at the same times
(probable over a limited area, like a large construction site?), and
calculate all the relative offsets between the points of interest and
the reference. How accurate would those offsets be? Especially
altitudes (important for things like sewer lines)?

I'm guessing pretty accurate, like down to the resolution of the
units. Say 6 inches for a GPS that reported minutes with 5 digits to
the right of the decimal point.

Thoughts?

--
Silvar Beitel

Sounds more like you are the CHEAPO EMPLOYER.(Send cheapo employee
around with the other unit to the points of interest in the site.)

On Sat, 30 Aug 2008 09:18:28 -0700 (PDT), Silvar Beitel wrote:

My brother the civil engineer asked me (the supposed family GPS
expert) about the following scenario (to help in developing initial
site plans without immediately having to go to expensive centimeter-
accurate surveying GPSs and/or bringing in and paying real surveyors,
which would come later on in the process):

Take a pair of cheap identical datalogging GPSs. Park one at a
reference point and let it sit there logging. Send cheapo employee
around with the other unit to the points of interest in the site.
Gather the two and dump out their logs. Match the timestamps, check
that both units used the same satellite set at the same times
(probable over a limited area, like a large construction site?), and
calculate all the relative offsets between the points of interest and
the reference. How accurate would those offsets be? Especially
altitudes (important for things like sewer lines)?

I'm guessing pretty accurate, like down to the resolution of the
units. Say 6 inches for a GPS that reported minutes with 5 digits to
the right of the decimal point.

Thoughts?

Interesting proposal. Sort of a poor man's differential GPS. I think it
would require a series of experiments to see how accurate this would be, as
a practical matter. Theoretically speaking, the horizontal distances might
be pretty good, but don't count on the altitude for anything approaching
accuracy. You'd be better off using recording barometers.

--
Mike Russell - http://www.curvemeister.com

Happy Trails wrote:

On Sat, 30 Aug 2008 09:18:28 -0700 (PDT), Silvar Beitel
wrote:

Thoughts?

If it worked, there would be no market for accurate (RTK) gps in the
construction industry.

If it worked, your cheapo brother would never bother to do the later
step with a real surveyor or accurate GPS.

What are you going to do as a two-step process anyway - set the
elevation of sewers, or location of curb cuts, hahaha?

Next you'll be asking if water really has to flow downhill all the
time.

That's an easy one, it doesn't.

Even rivers flow uphill, sometimes,

Jan

On Sat, 30 Aug 2008 09:18:28 -0700 (PDT), Silvar Beitel
wrote:

Thoughts?

If it worked, there would be no market for accurate (RTK) gps in the
construction industry.

If it worked, your cheapo brother would never bother to do the later
step with a real surveyor or accurate GPS.

What are you going to do as a two-step process anyway - set the
elevation of sewers, or location of curb cuts, hahaha?

Next you'll be asking if water really has to flow downhill all the
time.

"Silvar Beitel" wrote in message
...
Take a pair of cheap identical datalogging GPSs. Park one at a
reference point and let it sit there logging. Send cheapo employee
around with the other unit to the points of interest in the site.
Gather the two and dump out their logs. Match the timestamps, check
that both units used the same satellite set at the same times
(probable over a limited area, like a large construction site?), and
calculate all the relative offsets between the points of interest and
the reference. How accurate would those offsets be? Especially
altitudes (important for things like sewer lines)?

What exactly do you mean by a datalogging GPS? Even a $125 consumer gps
will keep a tracklog of thousands of points, but that does not include the
satellite info in the tracklog.

I have to agree with the other poster who said that it is unlikely to be
effective especially for elevation.


Easy to test though: Simply put two gps units outside far enough apart so
that you don't need to worry about interference between the two
(superheterodyne amplifiers put out a small rf signal). Five or ten feet is
probably more than enough. Then compare the two tracklogs over time. If
your system works, the distance between the two should remain constant and
match the distance measured with a tape measure. To make it even simpler
align the two units due N-S, so you can just look at the longitude and see
if they two units always show the same longitude and elevation.



--
Tom
http://home.att.net/~tbharvey/

On Aug 30, 3:01 pm, Mike Russell
wrote:

Interesting proposal. Sort of a poor man's differential GPS.

Exactly, except that with identical units (hardware and firmware rev.)
it might be even more accurate.

I think it
would require a series of experiments to see how accurate this would be, as
a practical matter.

Absolutely.

Theoretically speaking, the horizontal distances might
be pretty good, but don't count on the altitude for anything approaching
accuracy. You'd be better off using recording barometers.

Really? If unit A says it's at X units above the geoid and an
identical unit B says it's at Y, wouldn't the variation in X-Y be
pretty small over the site field?

Thanks for that input, Mike.

--
Silvar Beitel

On Aug 30, 5:01 pm, Happy Trails wrote:
On Sat, 30 Aug 2008 09:18:28 -0700 (PDT), Silvar Beitel

wrote:
Thoughts?

If it worked, there would be no market for accurate (RTK) gps in the
construction industry.

Valid point. Or, it hasn't been thought of in this particular context
(feasibility, very preliminary design).

If it worked, your cheapo brother would never bother to do the later
step with a real surveyor or accurate GPS.

What are you going to do as a two-step process anyway - set the
elevation of sewers, or location of curb cuts, hahaha?

Next you'll be asking if water really has to flow downhill all the
time.

Good engineering is an iterative process. Early plans of attack are
"soft" and not worth investing much money in. As feasibility morphs
into a concrete design and specifications, you invest more to insure
that your evolved design will work. A responsible engineer trades off
costs v. rewards at each stage.

--
Silvar Beitel

On Aug 30, 5:22 pm, "Tom H." wrote:
"Silvar Beitel" wrote in message

... Take a pair of cheap identical datalogging GPSs. Park one at a
reference point and let it sit there logging. Send cheapo employee
around with the other unit to the points of interest in the site.
Gather the two and dump out their logs. Match the timestamps, check
that both units used the same satellite set at the same times
(probable over a limited area, like a large construction site?), and
calculate all the relative offsets between the points of interest and
the reference. How accurate would those offsets be? Especially
altitudes (important for things like sewer lines)?

What exactly do you mean by a datalogging GPS? Even a $125 consumer gps
will keep a tracklog of thousands of points, but that does not include the
satellite info in the tracklog.

There is an abundance of GPS receivers on the market that simply
record time, position, etc.

Worst case, hook a sensor that outputs NMEA strings to a laptop and
just log all input to a file using a terminal emulation program.

I have to agree with the other poster who said that it is unlikely to be
effective especially for elevation.

Still not quite sure I get this, though. I know that vertical error
is higher than planar error, but is that true for the differential
case I'm talking about?

Easy to test though: Simply put two gps units outside far enough apart so
that you don't need to worry about interference between the two
(superheterodyne amplifiers put out a small rf signal). Five or ten feet is
probably more than enough. Then compare the two tracklogs over time. If
your system works, the distance between the two should remain constant and
match the distance measured with a tape measure. To make it even simpler
align the two units due N-S, so you can just look at the longitude and see
if they two units always show the same longitude and elevation.

Excellent suggestion.

--
Silvar Beitel

This is an idea that's being thrown out there for comment. How about beign
a little more receptive, and even having a little fun with the idea,
instead of just pouring scorn all over it? Sheesh.

--
Mike Russell - http://www.curvemeister.com