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[aprssig] traffic reporting

Jim Lux jimlux at earthlink.net
Wed Apr 6 17:37:38 UTC 2005


Steve wrote:

Looks like the sigalert site has the same constraints the one in South
Forida which preclude retransmission. The CalTrans site seems to have
the same raw info, but not presented in a concise form. It might be
possible to develop a screen-scraper, or maybe deep inside there is a
machine readable form of the data I didn't find in my short perusal.

---

The Caltrans site uses a java app on the client side, so there's probably 
some reasonably concise scheme for getting the data via HTTP.  I have 
contacted Caltrans about this.

---
Steve also wrote:
That image on the sigalert site is very
large, and would take a long time to transmit, if you wanted to get it
across in 5 minutes it would probably take the full 144.39 bandwidth.

---
12 kBytes to be exact (814x762 pixel GIF)..  98 kbits  ... If we assume we 
have about 200 bps total pipe bandwidth (1200bps less overhead at 20% 
channel utilization), and we want to be a "good citizen" and use no more 
than 20%.. we have 40 bps available.  Hmmm 2400 seconds... 40 minutes per 
image...

Clearly another approach is needed... a more efficient encoding, for one 
thing. (most of the image is fixed)..

----

Steve wrote:
Distilling it down to
segments of highway would take even less bandwith (probably a single 50
byte packet would do it) but receiving software would need to know the
location of each segment to display it properly (of course the proposed
solution of breaking down the image would also require new client
software).

---
That's sort of what I was thinking... could you define iconic short 
segments, which could be in one of 4 orientations (- \ | /) and perhaps 3 
colors (red, yellow, green)... APRS already deals with the position, so 
it's just a matter of displaying the short, fixed size, icons.

If you presume the basemap already exists, then the "little colored dot" 
approach on the sigalert site would work admirably.

I note that the colored diamond icons on the sigalert.com map are probably 
derived from the CHP database, and are georeferenced by thomas page and 
grid (and suffer from all the hideous inaccuracies of the underlying CHP 
reporting system, which requires an enormous amount of "interpretation")






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