[TowerTalk] HFTA Question

[Máximo EA1DDO_HK1H]

I am not an expert, but it seems they go down to 2m x 2m mesh resolution, and accuracy between 25 to 50 cm:

https://translate.google.co.uk/translate?sl=es&tl=en&u=https%3A%2F%2Fpnoa.ign.es%2Fespecificaciones-lidar

https://translate.google.co.uk/translate?sl=es&tl=en&u=http%3A%2F%2Fpnoa.ign.es%2Fmdt-lidar

73, Maximo


________________________________
De: TowerTalk <towertalk-bounces at contesting.com> en nombre de jimlux <jimlux at earthlink.net>
Enviado: viernes, 25 de septiembre de 2020 12:50
Para: towertalk at contesting.com <towertalk at contesting.com>
Asunto: Re: [TowerTalk] HFTA Question

On 9/24/20 11:44 PM, Máximo EA1DDO_HK1H wrote:
> Hi Bill,
>
> It seems that terrain model is not realistic, according to you.
> At my home country, Spain, we can download DEM up to 5m resolution, very high. Usually 25m is enough.

But don't confuse resolution with accuracy.

10m vertical accuracy is quite common with a resolution of 1 meter.
Likewise, the DEM may be 5m horizontal resolution, but the underlying
measurement might be only good to 30 meters or 10 meters (1 sigma).



> For worldwide coverage there are a couple of options.
> You can visit my web site talking about that>
> https://translate.google.com/translate?sl=es&tl=en&u=http%3A%2F%2Fwww.ea1ddo.es%2Fdem%2Fdem.html
>
> Worldwide DEM servers I know:
>
> ASTER  https://wist.echo.nasa.gov/wist-bin/api/ims.cgi?mode=MAINSRCH&JS=1

That link doesn't work, but this one might get you in the right area:

https://search.earthdata.nasa.gov/search?q=DEM


>
> SRTM  http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp

Note this comment:
Resampled SRTM data, spatial resolution approximately 250 meter on the
line of the equator, for the entire globe are available: (Click here)
Spatial resolution approximately 30 meter on the line of the equator:

So while the gridded data may be on 30 meter posts (1 arc second,
typically) it's interpolated from 250 meter resolution data (with >10m
uncertainty in vertical elevation - it's SRTM data)

>
> ATDI   http://www.atdi.com/cartography/
> <http://foro.ea1ddo.es/>
>
>
> Remember you can edit manually your data. I use to do it because HFTA only accepts 150 points, so i use to edit manually in a logaritmic way. The closer the points are, I take smaller separation, increasing when distance to antenna is larger. For example, I can leave one point every 5m from antenna up to 500m away, then every 25m, then every 50m, up to 150 points.
> It depends on your terrain profile, you need more detail where disrruptions are, less detail on flat land.
>
> 73, Maximo
>
>

from the GDEM Validation report:

The absolute vertical accuracy study found the GDEM2 to be within -0.20
meters on average when compared against 18,000 geodetic control points
over the CONUS, with an accuracy of 17 meters at the 95% confidence
level. The Japan study noted the GDEM2 differed from the 10-meter
national elevation grid by -0.7 meters over bare areas, and by
7.4 meters over forested areas. Similarly, the CONUS study noted the
GDEM2 to be about 8 meters above the 1 arc-second NED over most forested
areas, and more than a meter below NED over bare areas. The global
altimeter study found the GDEM2 to be on average within 3 meters of
altimeter-derived control, and also documented sensitivity to tree
canopy height. The Japan study noted that the horizontal displacement in
GDEM1 of 0.95 pixels was reduced to 0.23 pixels in GDEM2. Both teams
noted improvements in horizontal resolution, between 71 and 82 meters,
comparable to the SRTM 1 arc second elevation model, but at the cost of
some increased noise. The number of voids and artifacts noted in GDEM1
were substantially reduced in GDEM2, and in some areas virtually
eliminated.


Note the differences over forested areas (7.4 m vs -0.7m), and the
comment on horizontal resolution (around 75 meters, comparable to SRTM)
"at the cost of increased noise"

Noise, in a DEM, is, of course, seemingly random variations in surface
height.
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