What elevation tool do you use and how does it compare in accuracy?

I'm trying to do some antenna siting calculations ... and ... for
that ... I need elevation information.

However ... given any set of coordinates ... e.g., Mt Hamilton,
California at coordinates 37.337408,-121.644073 ... I find the following
elevation tools all give DIFFERENT elevations (some are off by more than
a hundred feet!)

1. Google Map API yields 1217.061889648438 meters (3992.985 feet)
http://maps.googleapis.com/maps/api/elevation/json?
locations=37.337408,-121.644073&sensor=false

2. Geoplaner yields 1217.1 meters (3993 feet)
http://www.geoplaner.com

2. Daftlogic yields 1230.988 meters (4038.676 feet)
http://www.daftlogic.com/sandbox-goo...d-altitude.htm

4. Earthtools yields 1210 meters (3969.8 feet)
http://www.earthtools.org

5. Heywhatsthat (SRTM db) yields 1213 meters (3980 feet)
http://www.heywhatsthat.com/profiler-0904.html

Q: Which elevation tool do you use ... & what's the accuracy?

Unless you are dealing with a surveyed peak, I would assume all the
elevation data is derived from USGS NEDs (national elevation dataset).
For the most part, they are only 1/3 arc second. I'm speculating that
some of these programs are reporting data from the NEDs as if they are
section of flat areas on the 1/3 arc second grid, while others are
interpreting the elevation using a weighting scheme.
http://seamless.usgs.gov/

Have you considered running SPLAT! ? It uses 1/3 arc second data. It can
predict line of sight. Generally I find I do better than the SPLAT!
prediction, so if SPLAT! says no, the answer is maybe, but if SPLAT!
says yes, you have line of sight. The program has hooks for predicting
signal strength, models knife edge diffraction, etc.
http://www.qsl.net/kd2bd/splat.html

Similar software is
http://www.cplus.org/rmw/english1.html

For pure line of sight, there is GRASS. However GRASS has a very steep
learning curve.
http://grass.osgeo.org/

What takes maybe 5 minutes on SPAT can take 2 days on GRASS. I don't
know if the results are that much better.

For the bay area, I suspect SPLAT! is fine. I think it's shortcomings
are in areas where there is really rugged terrain that probably isn't
modeled well.

On Sat, 3 Mar 2012 04:02:49 +0000 (UTC), alpha male
wrote:

What elevation tool do you use and how does it compare in accuracy?

I'm trying to do some antenna siting calculations ... and ... for
that ... I need elevation information.

However ... given any set of coordinates ... e.g., Mt Hamilton,
California at coordinates 37.337408,-121.644073 ... I find the following
elevation tools all give DIFFERENT elevations (some are off by more than
a hundred feet!)

1. Google Map API yields 1217.061889648438 meters (3992.985 feet)
http://maps.googleapis.com/maps/api/elevation/json?
locations=37.337408,-121.644073&sensor=false

2. Geoplaner yields 1217.1 meters (3993 feet)
http://www.geoplaner.com

2. Daftlogic yields 1230.988 meters (4038.676 feet)
http://www.daftlogic.com/sandbox-goo...d-altitude.htm

4. Earthtools yields 1210 meters (3969.8 feet)
http://www.earthtools.org

5. Heywhatsthat (SRTM db) yields 1213 meters (3980 feet)
http://www.heywhatsthat.com/profiler-0904.html

Q: Which elevation tool do you use ... & what's the accuracy?

Youre asking wrong questions.
First define a surface with height ZERO as a reference.
There are about 100 different definitions alone for that.
Distance from Earth center, median sea level at Novosibirsk,
or a San Francisco? Water isn't level, it follows gravitation.
And so on.

w.

On Sat, 03 Mar 2012 04:02:49 +0000, alpha male wrote:

What elevation tool do you use and how does it compare in accuracy?

I'm trying to do some antenna siting calculations ... and ... for that
... I need elevation information.

However ... given any set of coordinates ... e.g., Mt Hamilton,
California at coordinates 37.337408,-121.644073 ... I find the following
elevation tools all give DIFFERENT elevations (some are off by more than
a hundred feet!)

1. Google Map API yields 1217.061889648438 meters (3992.985 feet)
http://maps.googleapis.com/maps/api/elevation/json?
locations=37.337408,-121.644073&sensor=false

2. Geoplaner yields 1217.1 meters (3993 feet) http://www.geoplaner.com

2. Daftlogic yields 1230.988 meters (4038.676 feet)
http://www.daftlogic.com/sandbox-goo...d-altitude.htm

4. Earthtools yields 1210 meters (3969.8 feet) http://www.earthtools.org

5. Heywhatsthat (SRTM db) yields 1213 meters (3980 feet)
http://www.heywhatsthat.com/profiler-0904.html

Q: Which elevation tool do you use ... & what's the accuracy?

I'd use the info from another licensee's application, (on the same
tower). With the FCC you also must specify which survey you are using.
All towers over 200'(AIR) have their own entry in a tower database.

On Sat, 03 Mar 2012 00:10:32 -0800, miso wrote:
Have you considered running SPLAT!

I just installed the Splat (Surface Path Length And Terrain) RF analysis
application (version 1.3.0-1) from the default Ubuntu Software Center.
- http://joysofprogramming.com/install-splat-ubuntu/
- $ sudo apt-get install splat (dpkg -s splat)

It's apparently a command-line tool which needs me to download the
database so it may take a while to figure out.

Googling for a "splat!" tutorial, I find it's also a photoshop hack so
it's actually hard to figure out how to use it on the fly.

Q: Do you have a working example or two for how to use it from the
command line?

In rec.radio.amateur.antenna alpha male wrote:
What elevation tool do you use and how does it compare in accuracy?

I'm trying to do some antenna siting calculations ... and ... for
that ... I need elevation information.

However ... given any set of coordinates ... e.g., Mt Hamilton,
California at coordinates 37.337408,-121.644073 ... I find the following
elevation tools all give DIFFERENT elevations (some are off by more than
a hundred feet!)

1. Google Map API yields 1217.061889648438 meters (3992.985 feet)
http://maps.googleapis.com/maps/api/elevation/json?
locations=37.337408,-121.644073&sensor=false

2. Geoplaner yields 1217.1 meters (3993 feet)
http://www.geoplaner.com

2. Daftlogic yields 1230.988 meters (4038.676 feet)
http://www.daftlogic.com/sandbox-goo...d-altitude.htm

4. Earthtools yields 1210 meters (3969.8 feet)
http://www.earthtools.org

5. Heywhatsthat (SRTM db) yields 1213 meters (3980 feet)
http://www.heywhatsthat.com/profiler-0904.html

Q: Which elevation tool do you use ... & what's the accuracy?

GPS 3 meter error

On Sat, 03 Mar 2012 15:59:39 +0000, alpha male wrote:



splat | less



http://www.kgiwireless.com/Documents...icSiteList.asp

http://www.americantower.com/sitelocator/default.aspx

On 3/3/2012 7:59 AM, alpha male wrote:
On Sat, 03 Mar 2012 00:10:32 -0800, miso wrote:
Have you considered running SPLAT!

I just installed the Splat (Surface Path Length And Terrain) RF analysis
application (version 1.3.0-1) from the default Ubuntu Software Center.
- http://joysofprogramming.com/install-splat-ubuntu/
- $ sudo apt-get install splat (dpkg -s splat)

It's apparently a command-line tool which needs me to download the
database so it may take a while to figure out.

Googling for a "splat!" tutorial, I find it's also a photoshop hack so
it's actually hard to figure out how to use it on the fly.

Q: Do you have a working example or two for how to use it from the
command line?

Quote:

$ splat --help *** ERROR: No transmitter site(s) specified! $ splat --==[ SPLAT! v1.3.0 Available Options... ]==-- -t txsite(s).qth (max of 4 with -c, max of 30 with -L) -r rxsite.qth -c plot coverage of TX(s) with an RX antenna at X feet/meters AGL -L plot path loss map of TX based on an RX at X feet/meters AGL -s filename(s) of city/site file(s) to import (5 max) -b filename(s) of cartographic boundary file(s) to import (5 max) -p filename of terrain profile graph to plot -e filename of terrain elevation graph to plot -h filename of terrain height graph to plot -H filename of normalized terrain height graph to plot -l filename of path loss graph to plot -o filename of topographic map to generate (.ppm) -u filename of user-defined terrain file to import -d sdf file directory path (overrides path in ~/.splat_path file) -m earth radius multiplier -n do not plot LOS paths in .ppm maps -N do not produce unnecessary site or obstruction reports -f frequency for Fresnel zone calculation (MHz) -R modify default range for -c or -L (miles/kilometers) -db threshold beyond which contours will not be displayed -nf do not plot Fresnel zones in height plots -fz Fresnel zone clearance percentage (default = 60) -gc ground clutter height (feet/meters) -ngs display greyscale topography as white in .ppm files -erp override ERP in .lrp file (Watts) -ano name of alphanumeric output file -ani name of alphanumeric input file -udt name of user defined terrain input file -kml generate Google Earth (.kml) compatible output -geo generate an Xastir .geo georeference file (with .ppm output) -dbm plot signal power level contours rather than field strength -gpsav preserve gnuplot temporary working files after SPLAT! execution -metric employ metric rather than imperial units for all user I/O If that flew by too fast, consider piping the output through 'less': splat | less Type 'man splat', or see the documentation for more details. This compilation of SPLAT! supports analysis over a region of 8 square degrees of terrain.

SPLAT! isn't a photoshop hack, but you can use gimp (or I suppose
photoshop) to hack with the png files it creates. I take the png files
and chop them up so they are acceptable to google earth, then make an
overlay out of them. Splat can do it, but unless things have changed, it
makes a kml file that can be too large for GE to accept. GE wants tiled
imagery. I haven't run it in maybe 18 months, so I can't really say much
without setting it up again.

I saw a GPS mentioned. I never found GPS elevation to be too accurate. I
have a barometer in my GPS. I never used it, so I can't vouch for it's
accuracy. But you can enter in the pressure from the airport, or
calibrate it to a known reference (known altitude for a position).

One idea would be to check the USGS monuments and find the closest
reference.
http://www.ngs.noaa.gov/cgi-bin/datasheet.prl

Note that sometimes these markers are on private property. Other times
they are in the middle of the street! it pays to look at the reference
on google earth before trying to use it. My Garmin gps60cs was good to
4ft, basically one lsb.

You could get a nearby reference, call the barometer, then quickly move
to your spot before the pressure changes. Or you might get lucky and
find there is a marker there already.

Topo maps have lines of constant altitude, usually on 20ft contours. You
could interpolate from the map.

If you really need accurate data, just pay for a survey. I would guess
something that simple is under a grand. I've paid for land surveys and
they are a few grand, but the altitude at one point is pretty simple.
Potentially the civil engineering firm might have topo data on file that
is not available to the general public. For instance, I paid for a
satellite topo to be done on some property. The civil engineering firm
has it on file and I'm sure it gets peeked at by other people.

On Sat, 03 Mar 2012 09:13:45 +0100, Helmut Wabnig [email protected] ---
-.dotat wrote:

Youre asking wrong questions.
First define a surface with height ZERO as a reference.
There are about 100 different definitions alone for that.
Distance from Earth center, median sea level at Novosibirsk,
or a San Francisco? Water isn't level, it follows gravitation.
And so on.

w.

If in the US, I would assume that the LAT/LONG uses either WGS84,
NAD27, or NAD83 datums. That reduces the number of available options.

Unfortunately, the USGS is still hanging onto NAD27, while most
mapping programs and displays are on WGS84.
http://www.maptools.com/UsingUTM/mapdatum.html
In the People's Republic of Santa Cruz, the error is about 20 meters
east-west, and about 1 meter north-south. I forgot which way. The
problem becomes really bad when trying to locate a mountain top.
20-200 meters of horizontal error can easily move a position from the
peak, to somewhere on the slope, resulting in large altitude errors.

SRTM and SRTM2 are another oddity. They were created from the space
shuttle, using a radar altimeter. Depending on the whether it's
looking at buildings or trees, there's no really good way to determine
of the indicated altitude is the top of a 100ft redwood tree, the top
of a 10 meter high building, or at ground level.

So, my list of rhetorical questions a
1. What is the OP trying to accomplish? If for an FCC license HAAT
calculation, almost any reasonable guess will suffice.
http://transition.fcc.gov/mb/audio/bickel/haat_calculator.html
If for doing Radio-Mobile coverage contours, you'll need to use the
built in mapping tool to find the peak or exact location on the
mountain top. The choice of application depends on what one is
attempting to accomplish.
2. What level of accuracy is really required? The original position
of 37.337408N -121.644073W is specified to 1 millionth of a degree, or
about 0.09 meters.
http://www.csgnetwork.com/degreelenllavcalc.html
It would be interesting to know where this highly accurate number came
from. Better GPS receivers, without DGPS, can do 3 meters accuracy.
3. What datum are you using? I suggest WGS84. If the lookup tool
offers a choice of datums, pick one and stay with it.
4. Are you interested in ground level, building rooftop level, or
tree top level?

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com
# http://www.LearnByDestroying.com AE6KS

On Sat, 3 Mar 2012 18:47:56 -0000, wrote:

GPS 3 meter error

Yep. However, standing under the tower, in order to take advantage of
this level of accuracy is often impractical. Climbing barbed wire
topped chain link fences is not my idea of fun. I ran into the
problem when I was throwing together a map of the local cell sites:
http://802.11junk.com/cellular/ (from about 2002).
I had a suitable GPS, but I couldn't get anywhere near some of the
towers in order to get an accurate position. I had to record a GPS
location nearby, measure or estimate the distance and direction to the
tower, and then do the math.

I forgot to mumble something about HARN and NAVD88, if the best
possible vertical accuracy is required:
http://www.ngs.noaa.gov/faq.shtml

Also, all the major mount tops have benchmarks pounded into the ground
somewhere. If you can find the benchmark, you have the location, and
sometimes the altitude. For example, for Mt Hamilton:
http://www.waymarking.com/waymarks/WMQ1J_Mt_Hamilton_1960_ECC
The problem is that mountains tend to move every time there's an
earthquake. Even without earthquakes, the distance between Mt
Hamilton and the Farallan Islands moves about 5 cm per year across the
San Andreas fault.

I think this might help if you're trying to locate a tower on Mt
Hamilton:
http://www.ucolick.org/~sla/mtham/gpos.html
"The vertical positions are not as well known and may
be off by 10 meters. "
Well, maybe not...


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558