[TowerTalk] Terrain data for HFTA

[Michael Tope]

Using the Radio Mobile program, I found the SRTM 1 arcsecond elevation 
data to match very well with GPS readings I took in the high desert area 
just north of Los Angeles (to within a few feet typically). The 1 
arcsecond data set has a pixel size of about 30 meters x 30 meters, so 
for any given pixel the reported elevation is the average of the 
elevation over that pixel. For gently sloped terrain this average 
elevation should be a very good approximation. In areas where the 
elevation changes rapidly over the pixel area, there will be more 
disagreement between the elevation reported in the SRTM data and the 
actual elevation. IIRC the requirement on the SRTM instrument was 16 
meter absolute elevation accuracy and 10 meter relative accuracy, but 
the folks I recall talking to who worked on the instrument were elated 
that the performance was typically much better than the minimum 
requirements.

73, Mike W4EF

On 8/20/2011 5:00 AM, "Felipe J. Hernández" wrote:
> Dean,
>
> Me too would like to take that data from google.  In my case ive found
> that the data
> from SRTM is never true to my terrain. I trust google earth because it
> gives me the
> exact contour and height.
>
> For me extracting from google was a little painful but rewarding, after
> i remodeled the terrain
> with the information, my antennas and real life experience on the bands
> started to make sense.
>
> I took samples every 10 ft very close to the antennas and then moved to
> 25 ft.
>
> Felipe
> NP4Z
>
>
> On 8/17/2011 2:29 PM, RDStraw wrote:
>> Charlie:
>>
>> You said recently:
>>
>>> Date: Tue, 16 Aug 2011 20:12:45 -0500
>>> From: Charlie Carroll<k1xx at cfl.rr.com>
>>> Subject: Re: [TowerTalk] grabbing terrain data for HFTA
>>> To: Grant Saviers<grants2 at pacbell.net>, towertalk at contesting.com
>>> Message-ID:<wg1l6uo0lqvcmp2rougptbc7.1313543269277 at email.android.com>
>>> Content-Type: text/plain; charset=utf-8
>>>
>>> Door's abt to close, so have to be brief.  Of late, I have taken
>>> to semi-automatically extracting terrain data from Google Earth.
>>> I dump this info into an Excel spreadsheet for the number
>>> crunching.  Pretty painless to do something like ten foot
>>> intervals out to 10k feet.
>>>
>>> 73 charlie, k1xx
>> N6BV: I'm not sure where Google Earth gets their terrain data, but I am
>> suspicious that it is derived at least partially from the SRTM (Shuttle
>> Radar Topography Mission) data set. I get nervous when I look at the terrain
>> data for islands surrounded by seawater -- which should, by definition, be
>> at or at least very close to sea level. It is often shown as being
>> considerably different from sealevel.
>>
>> N6BV: For example, in Google Earth zoom into Baileytown, Bimini, Bahamas.
>> Just to the west of Baileytown the altitude goes considerably negative.
>> Either this is a depthsounder reading (which I doubt) or else something is
>> goofy!
>>
>> N6BV: I'm curious how you are extracting that data from Google Earth
>> semi-automatically, even if I'm nervous about the data itself.
>>
>>> Grant Saviers<grants2 at pacbell.net>   wrote:
>>>
>>>> Can some one share a pointer for up to date instructions for grabbing
>>>> the terrain data from the web for HFTA?  Every instruction I've found is
>>>> hopelessly out of sync with the current terrain data web site.
>>>>
>>>> Grant KZ1W
>>> Date: Wed, 17 Aug 2011 06:24:03 -0400
>>> From: Pete Smith<n4zr at contesting.com>
>>> Subject: Re: [TowerTalk] grabbing terrain data for HFTA
>>> To: Charlie Carroll<k1xx at cfl.rr.com>
>>> Cc: towertalk at contesting.com, Grant Saviers<grants2 at pacbell.net>
>>> Message-ID:<4E4B96C3.5000904 at contesting.com>
>>> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>>>
>>> Charlie, this sounds like an excellent solution - potentially much less
>>> involved than the old way of using MicroDem and the USGS website.  If
>>> you can provide a few more details, I'll be glad to write it up, and I'd
>>> bet that N6BV and the ARRL will be interested in making it available on
>>> the ARRL web-site.  Dean and I wrote most of the old instructions you'll
>>> find, but it always was a PITA because USGS kept changing their web-site.
>>>
>>> 73, Pete N4ZR
>> Grant/Pete:
>>
>> N6BV: This is the latest operating instructions for HFTA, dated Aug 3, 2009.
>> I checked the links for obtaining DEM, NED and SRTM data and they all work.
>>
>> http://www.arrl.org/files/file/Product%20Notes/Antenna%20Book/HFTA.pdf
>>
>> Grant: It may be useful for you to check out a webinar I gave. It's entitled
>> "Hinks and Kinks for Using HFTA" and can be found on the PVRC website:
>>
>> http://www.pvrc.org/webinar/webinars.htm
>>
>> 73, Dean, N6BV
>>
>>