Re: [time-nuts] LTE-Lite
Do not trust Google Earth data for any precision work. The mentioned six
feet are probably due to the geographical data, not to the precission of
your GPS unit. If you look for image seams you can verify the kind of
errors involved.
How good are USGS topo maps for this sort of thing? Most streets are shown
as a pair of parallel lines, but the separation of the lines doesn't match
the actual width of most streets. Does the center of that pair on the paper
correspond to the center of the road? Can I use the intersection of a pair
of streets as a reference point? ...
How about equivalent maps for other countries?
How well do typical benchmarks agree with GPS?
Are the surveyors maps used for deeds useful in this context?
--
These are my opinions. I hate spam.
On 11/30/14, 1:49 PM, Hal Murray wrote:
Do not trust Google Earth data for any precision work. The mentioned six
feet are probably due to the geographical data, not to the precission of
your GPS unit. If you look for image seams you can verify the kind of
errors involved.
How good are USGS topo maps for this sort of thing?
USGS National Map Accuracy Standards are 1/50th of an inch at map scale
(essentially, the width of a pencil line). That is, things on the map
are within 1/50th of an inch where they actually are. So, on a
1:250,000 map, one can expect 100 meter errors. On a 1:24,000 map, 10
meter errors, etc.
Most streets are shown
as a pair of parallel lines, but the separation of the lines doesn't match
the actual width of most streets.
That's the symbology: that is, you're seeing a map symbol for a street
of a particular class, not the actual dimensions of the street.
Does the center of that pair on the paper
correspond to the center of the road? Can I use the intersection of a pair
of streets as a reference point? ...
No.
What you could use is the center point for Bench Marks (BM) on the map,
variously represented as crosses or triangles. And, of course, they're
only accurate to 0.02 inches on the map (0.0508 cm).
In practice, most USGS maps are somewhat better than this, assuming you
allow for things like changes since the map revision date. My house is
moving at roughly 1-2cm/year due to tectonic motion, and, so, a map that
was revised in 1980 will be some 30-60cm in error. Since a 1:24,000 map
is the standard 7.5 minute quad, there can be 12 meter uncertainty, and
the map doesn't need to be updated.
ANd then we get into map datums. Is your map NAD27 or WGS84?
How about equivalent maps for other countries?
How well do typical benchmarks agree with GPS?
Are the surveyors maps used for deeds useful in this context?
On 30/11/2014 a las 22:49, Hal Murray wrote:
Do not trust Google Earth data for any precision work. The mentioned six
feet are probably due to the geographical data, not to the precission of
your GPS unit. If you look for image seams you can verify the kind of
errors involved.
How good are USGS topo maps for this sort of thing? Most streets are shown
as a pair of parallel lines, but the separation of the lines doesn't match
the actual width of most streets. Does the center of that pair on the paper
correspond to the center of the road? Can I use the intersection of a pair
of streets as a reference point? ...
How about equivalent maps for other countries?
How well do typical benchmarks agree with GPS?
Are the surveyors maps used for deeds useful in this context?
In fact I'm not familiar with USGS topo maps, but here in Spain for the
small scales(Scale 1:25000 and lower) the streets and roads are not wide
enough to be accurately represented, so a symbol is used instead. The
symbol style is selected to mean the type of road and yes, the center of
the parallel lines corresponds to the center of the street. Another
thing is the overall precision, here it is established that the
precision of the paper maps should be equal to the unaided eye
resolution, about 1/4 mm, so you multiply .25 mm times the scale
denominator and you get the precision. Our main national topographic map
is at 1:25000 scale and its precision is about 6.25 meters. For digital
maps the precision is what the map provider says, since a digital map
can be "enlarged" at will. Usually the precision is consistent with the
intended representation scale in the same terms as the paper maps.
Since Google's geographical data is usually obtained from official
sources, the line maps are quite good, but the satellite images usually
are not very well rectified and stitched, unless they are obtained from
similar sources which put a lot of effort on its accuracy and matching
with the maps. This varies a lot depending the region.
The surveyor maps usually agree very well with GPS, in fact they are
based in GPS measured reference points these days.
This comes from my limited experience, the results can be very different
depending the zone and the date.
Best regards,
Ignacio EB4APL
With the LTE-Lite, are the survey results held in non-volatile memory, or
does it need to do a survey each time it is switched on? This is a fixed
location.
The survey light is still on after a number of hours of operation (but it
may have gone off in the meanwhile), and the GPS light sometimes flashes and
sometimes not. The signals I'm seeing at the moment are: 24 30 26 18 32 27
21 18 and 19 in the "signal quality" indicator of Visual GPS. This with the
puck on the top storey of a two storey building, but indoors. Other GPS
pucks work fine in the same location. The PPS output appears to be correct,
and there is 20 MHz from the 20 MHz port, but nothing from the Clock Out
port on this 10 MHz unit.
The unit is as-received, with the exception of switching to NMEA sentences.
Thanks,
David
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor@blueyonder.co.uk
David it always does a survey. Though even while doing that the frequency
output is fine after its had a bit to stabilize. I wanted to bring the
survey lamp out to a front panel LED however that appeared to be more work
and risk then the value.
The documentation says that from time to time it will do a re-survey.
Frankly my units racked and stacked with dividers filters and line drivers
for various frequencies I am using and its running very very smoothly.
By the way at a huge power consumption of 1-3 watts. The power mete doesn't
read well at this level.
Regards
Paul
WB8TSL
On Thu, Dec 4, 2014 at 11:07 AM, David J Taylor <
david-taylor@blueyonder.co.uk> wrote:
With the LTE-Lite, are the survey results held in non-volatile memory, or
does it need to do a survey each time it is switched on? This is a fixed
location.
The survey light is still on after a number of hours of operation (but it
may have gone off in the meanwhile), and the GPS light sometimes flashes
and sometimes not. The signals I'm seeing at the moment are: 24 30 26 18
32 27 21 18 and 19 in the "signal quality" indicator of Visual GPS. This
with the puck on the top storey of a two storey building, but indoors.
Other GPS pucks work fine in the same location. The PPS output appears to
be correct, and there is 20 MHz from the 20 MHz port, but nothing from the
Clock Out port on this 10 MHz unit.
The unit is as-received, with the exception of switching to NMEA sentences.
Thanks,
David
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor@blueyonder.co.uk
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David it always does a survey. Though even while doing that the frequency
output is fine after its had a bit to stabilize. I wanted to bring the
survey lamp out to a front panel LED however that appeared to be more work
and risk then the value.
The documentation says that from time to time it will do a re-survey.
Frankly my units racked and stacked with dividers filters and line drivers
for various frequencies I am using and its running very very smoothly.
By the way at a huge power consumption of 1-3 watts. The power mete doesn't
read well at this level.
Regards
Paul
WB8TSL
---=========
Thanks, Paul. I had rather hoped that there might be some EEPROM in one of
the chips where the data was stored, oh well! I wonder just how long "a bit
to stabilize" takes? I might have mine on 24 x 7, but I might not...
My survey LED has still not gone out despite the unit being on overnight,
and the Alarm LED is lit, so I wish there was a way of relaxing the survey
constraints a little to get the survey complete. Viewing the NMEA output
with either Visual GPS or the U-blox software suggests that, despite good
signals, the unit is only getting lock half the time. The positions it
produces are correct. I read in the documentation that the serial/USB data
is output only, though.
(I was wrong on the 10 MHz output being 20 MHz, I forget my other reference
was 5 MHz.)
73,
David GM8ARV
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor@blueyonder.co.uk
On 5 Dec 2014 07:05, "David J Taylor" david-taylor@blueyonder.co.uk wrote:
David it always does a survey. Though even while doing that the frequency
output is fine after its had a bit to stabilize. I wanted to bring the
survey lamp out to a front panel LED however that appeared to be more work
and risk then the value.
Have you considered to use a light pipe? Hopefully you could get enough
light out. Or is all else fails, use a photodiode to detect the light and
drive an LED.
Dave.
Have you considered to use a light pipe? Hopefully you could get enough
light out. Or is all else fails, use a photodiode to detect the light and
drive an LED.
Dave.
---===
Good suggestion, Dave.
Light pipes used to be very popular, but I couldn't find one when I searched
a little while back. Perhaps I was using the wrong search terms! I would
have thought that Maplin, for example, would have something.
David
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor@blueyonder.co.uk
I think the name "light pipe" has been supplanted by fiber-optic.
-Chuck Harris
David J Taylor wrote:
Have you considered to use a light pipe? Hopefully you could get enough
light out. Or is all else fails, use a photodiode to detect the light and
drive an LED.
Dave.
---===
Good suggestion, Dave.
Light pipes used to be very popular, but I couldn't find one when I searched a little
while back. Perhaps I was using the wrong search terms! I would have thought that
Maplin, for example, would have something.
David
On 5 Dec 2014 13:19, "Chuck Harris" cfharris@erols.com wrote:
I think the name "light pipe" has been supplanted by fiber-optic.
-Chuck Harris
Technically I agree that they have a lot in common. But I think the large
devices, which are often not cylindrical, are usually called light pipes.
http://uk.mouser.com/Mobile/Optoelectronics/LED-Indication/LED-Light-Pipes/_/N-b1d20
Some light pipes are hollow inside. I think that is stretching the
definition of optical fibre.
According to Wikipedia, light pipes or light tubes were originally
developed by the ancient Egyptians.
Some of these things are hollow are more than 1 m in diameter. I would
hardly call that an optical fibre.
But call them what you fancy (optical fibre, multi more fibre, waveguide,
light tube, light pipe. ...) I think such a device might solve the problem
getting the LTE Lite's status LEDs onto a box.
I don't have an LTE Lite, but given that they are low power devices, where
heat generation is undesirable, I suspect that the light output level
might be a bit low. In which case a photodiode or similar may be needed.
Dave.