Oops! Biggin Hill - Ground Height (SRTM and OS), Aerial Height

I check public usage of the Terrestrial Calculator most nights,
including choosing Find The Likeliest for any UK location given, to
check the prediction is sensible. Tonight, for some unfortunate user
in Spain, it wasn't:
http://www.macfh.co.uk/PrivTest/BigginHill.png

I've been aware for a month or so, ever since discovering in a similar
way that the SRTM tiles for The Shetlands gave out above 60N and
having to make my own replacement ones from OS data to cover them,
that SRTM heights and OS heights do not always agree, but have been
wondering what I may need to do about it. However this is the first
time that I've actually encountered an absurdity arising from it.

What has happened here is that the height of the Biggin Hill Antenna
is from OS at Terrain Height 153m plus Structure Height 17m giving a
total height of 167m - all sources that I've been able to find seem
to agree on this. However SRTM thinks the ground height is 177m which
should give an antenna height of 194m. I suspect the geoids used to
ground the two heights are different anyway, but hadn't suspected that
the difference in them would be significant. Clearly at least I'll
need to look into that more thoroughly now.

Before I think about rewriting the entire calculator to use OS heights
(which would probably necessitate converting the entire country's data
to a different file format to maintain the current speed of the
calculations), does anybody happen to know whether the official height
given for Biggin Hill is actually correct?

Note, I do mean Biggin Hill, not Biggin Hill Link.
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Please always reply to ng as the email in this post's
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I'm thoroughly confused now.
Brian

--
Brian Gaff -
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
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"Java Jive" wrote in message
...
I check public usage of the Terrestrial Calculator most nights,
including choosing Find The Likeliest for any UK location given, to
check the prediction is sensible. Tonight, for some unfortunate user
in Spain, it wasn't:
http://www.macfh.co.uk/PrivTest/BigginHill.png

I've been aware for a month or so, ever since discovering in a similar
way that the SRTM tiles for The Shetlands gave out above 60N and
having to make my own replacement ones from OS data to cover them,
that SRTM heights and OS heights do not always agree, but have been
wondering what I may need to do about it. However this is the first
time that I've actually encountered an absurdity arising from it.

What has happened here is that the height of the Biggin Hill Antenna
is from OS at Terrain Height 153m plus Structure Height 17m giving a
total height of 167m - all sources that I've been able to find seem
to agree on this. However SRTM thinks the ground height is 177m which
should give an antenna height of 194m. I suspect the geoids used to
ground the two heights are different anyway, but hadn't suspected that
the difference in them would be significant. Clearly at least I'll
need to look into that more thoroughly now.

Before I think about rewriting the entire calculator to use OS heights
(which would probably necessitate converting the entire country's data
to a different file format to maintain the current speed of the
calculations), does anybody happen to know whether the official height
given for Biggin Hill is actually correct?

Note, I do mean Biggin Hill, not Biggin Hill Link.
--
================================================== =======
Please always reply to ng as the email in this post's
header does not exist. Or use a contact address at:
http://www.macfh.co.uk/JavaJive/JavaJive.html
http://www.macfh.co.uk/Macfarlane/Macfarlane.html

On Thu, 8 Dec 2011 10:55:44 -0000
"Brian Gaff" wrote:

I'm thoroughly confused now.
Brian


Your top-posting and inclusion of the message you are replying to
inside your sig. is also confusing.
--
Davey.

On Thu, 8 Dec 2011 10:58:58 +0000, Davey wrote:

Your top-posting and inclusion of the message you are replying to
inside your sig. is also confusing.

Brian is blind and it's much easier for him to top post.

--
Alan White
Mozilla Firefox and Forte Agent.
By Loch Long, twenty-eight miles NW of Glasgow, Scotland.
Webcam and weather:- http://windycroft.co.uk/weather

On 07/12/2011 22:30, Java Jive wrote:
I check public usage of the Terrestrial Calculator most nights,
including choosing Find The Likeliest for any UK location given, to
check the prediction is sensible. Tonight, for some unfortunate user
in Spain, it wasn't:
http://www.macfh.co.uk/PrivTest/BigginHill.png

I've been aware for a month or so, ever since discovering in a similar
way that the SRTM tiles for The Shetlands gave out above 60N and
having to make my own replacement ones from OS data to cover them,
that SRTM heights and OS heights do not always agree, but have been
wondering what I may need to do about it. However this is the first
time that I've actually encountered an absurdity arising from it.

What has happened here is that the height of the Biggin Hill Antenna
is from OS at Terrain Height 153m plus Structure Height 17m giving a
total height of 167m - all sources that I've been able to find seem
to agree on this. However SRTM thinks the ground height is 177m which
should give an antenna height of 194m. I suspect the geoids used to
ground the two heights are different anyway, but hadn't suspected that
the difference in them would be significant. Clearly at least I'll
need to look into that more thoroughly now.

Before I think about rewriting the entire calculator to use OS heights
(which would probably necessitate converting the entire country's data
to a different file format to maintain the current speed of the
calculations), does anybody happen to know whether the official height
given for Biggin Hill is actually correct?

Note, I do mean Biggin Hill, not Biggin Hill Link.


Site height for Biggin Hill is 153m aod and antenna height is 14m agl,
at least according to my 1998 copy of the transmitter database.

Phil

That's because you can't see the picture, Brian. The total antenna
height is based on a terrain height from OS of 153m, whereas the
ground elevation comes from the Shuttle Radar Topography Mission
(SRTM), which gives 177m. As the difference is greater than the mast
height, the signal from the transmitter appears to be issuing from
within the ground.

On Thu, 8 Dec 2011 10:55:44 -0000, "Brian Gaff"
wrote:

I'm thoroughly confused now.
--
================================================== =======
Please always reply to ng as the email in this post's
header does not exist. Or use a contact address at:
http://www.macfh.co.uk/JavaJive/JavaJive.html
http://www.macfh.co.uk/Macfarlane/Macfarlane.html

Well Ofcom ...
http://www.ofcom.org.uk/static/recep...gion_id=3.html
.... and MB21 ...
http://tx.mb21.co.uk/gallery/gallerypage.php?txid=32
.... appear to agree with your site height, but think the antenna
height is 17m.

But if it really is 14m, I'm in an even bigger hole!

On Thu, 08 Dec 2011 16:31:00 +0000, phil
wrote:

Site height for Biggin Hill is 153m aod and antenna height is 14m agl,
at least according to my 1998 copy of the transmitter database.
--
================================================== =======
Please always reply to ng as the email in this post's
header does not exist. Or use a contact address at:
http://www.macfh.co.uk/JavaJive/JavaJive.html
http://www.macfh.co.uk/Macfarlane/Macfarlane.html

On 08/12/2011 18:38, Java Jive wrote:
Well Ofcom ...
http://www.ofcom.org.uk/static/recep...gion_id=3.html
... and MB21 ...
http://tx.mb21.co.uk/gallery/gallerypage.php?txid=32
... appear to agree with your site height, but think the antenna
height is 17m.

But if it really is 14m, I'm in an even bigger hole!

On Thu, 08 Dec 2011 16:31:00 +0000,
wrote:

Site height for Biggin Hill is 153m aod and antenna height is 14m agl,
at least according to my 1998 copy of the transmitter database.

It's also 14m in my 2007 DSO planning database. The total height of the
mast may be 17m, but the pictures on MB21 shows that the transmit
antenna is somewhat lower.

Phil

Doh! You're right: 153 + 14 = 167, as given by Ofcom.

On Thu, 08 Dec 2011 19:39:54 +0000, phil
wrote:

It's also 14m in my 2007 DSO planning database. The total height of the
mast may be 17m, but the pictures on MB21 shows that the transmit
antenna is somewhat lower.
--
================================================== =======
Please always reply to ng as the email in this post's
header does not exist. Or use a contact address at:
http://www.macfh.co.uk/JavaJive/JavaJive.html
http://www.macfh.co.uk/Macfarlane/Macfarlane.html

Your pedantic post is also boring, but I won't hod it against you.
Brian


--
Brian Gaff -
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"Davey" wrote in message
...
On Thu, 8 Dec 2011 10:55:44 -0000
"Brian Gaff" wrote:

I'm thoroughly confused now.
Brian


Your top-posting and inclusion of the message you are replying to
inside your sig. is also confusing.
--
Davey.