I am currently playing around with the Python version, but to solve
the following problem would be prepared to use the Java version if
need be.
I want to use elevation data to work out for a given UK (and world,
with less functionality) location which are the most likely TV
transmitters, and to display signal profile data. I already have a
working page here:
http://www.macfh.co.uk/JavaJive/AudioVisualTV/TerrestrialTV/TerrestrialCalculator.html
However, that page reads elevation data from a GAE set up by a friend
in the Ordnance Survey forum, and has some existing and potential
disadvantages:
- The existing GAE data only covers UK mainland, not other areas
covered by UK transmissions such as CI, NI, IoM, etc.
- All the signal computation is done client-side by JavaScript, making
it necessary for the calculations to be simplified, for example ...
- Clutter and transmitter directional coverage data have had to be
omitted.
- A risk that, if and as my page gets popular, the demand from it will
start to affect other users of his GAE.
Consequently, I have been looking at alternatives ...
Firstly, I looked at using the Elevation Service from Google Maps v3,
and have such a working version of the page on my PC. However,
getting sufficient data from the v3 ES in a short space of time tends
to fall foul of usage limits, and the daily limits are also quite low
for this purpose, but the real killer at the moment is that v3 maps do
not print well in *ANY* of the major browsers, and this is totally
unacceptable to me.
So now I am looking at a GAE of my own ...
Some time ago, my friend from the OS forum sent me his application
source files so that I could play around with the App to see how it
works. This has been most useful, but has convinced me that I need to
change the way the data is stored.
Ideally, I think I want to be able to store the UK SRTM data unzipped
as static files for the app - there would be a little over 100
covering the entire British Isles including CI, Eire & NI, IoM, at
2.818KB each makes about 300Mb. Then for the reduced functionality
for outside the UK, I would actually go as needed to the SRTM site and
get whatever files are needed on the fly, and these would have to be
unzipped on the fly, before being read into the app to be used exactly
as the UK data would have been, except that only one signal profile
will ever need to be fetched at one time.
So I need to know ...
1) How to read a static file as binary data
There is an example in the Google docs which looks promising ...
m.blob = db.Blob(open("image.png").read())
... but would I be able to extract (a) particular byte(s) starting at
a particular offset into the binary data after it has been read into
the blob?
2) Could I similarly fetch and read a zipped file using the URL Fetch
Python API?
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