Re: FAA To Finally Ditch Floppy Disks & Win-95

On Sun, 22 Jun 2025 21:55:47 GMT, Charlie Gibbs wrote:

I've written call accounting software for local 911 centres. The
switches give us latitude and longitude in degrees with six decimals.
That narrows it down to about 4 inches, which seems a bit optimistic,
unless you somehow have access to a differential GPS setup. 

Back in the bad old days of K&E slide rules, even with magnifying cursors,
you learned to do some sanity checking. Six decimal accuracy with consumer
grade GPS receivers that have 5 to 10 meter accuracy on the best of days
doesn't pass the sanity check.

As AVL systems in the equipment became more prevalent we'd get a variety
of formats, NMEA sentences being a popular one.

https://w3.cs.jmu.edu/bernstdh/web/common/help/nmea-sentences.php

Most of the USB hockey puck receivers spit them out and we'd wait for a
GGA or RMC sentence.

Years ago Trimble was a big player when emergency services started using
AVL and they had their own format. Geocom, another player, sent the
coordinated in hex. It's been a long time but there was also a magic
number involved where x and y were the hex values.

*lat = (double) x * 5.729577957 * .0000001;
*lon = (double) y * 5.729577957 * .0000001;

CalAmp sends a very proprietary struct with the values packed in big
endian format.  Great fun.

Anyway, it all ended up as decimal degrees. Th job wasn't finished since
most of the GIS data was in State Plane Coordinate System.

https://en.wikipedia.org/wiki/State_Plane_Coordinate_System

Like the article say, having Cartesian coordinates makes the math a lot
easier. With spherical coordinates to find the distance from A to B you
need to use the haversine or Vincenty's  formula.

https://en.wikipedia.org/wiki/Vincenty's_formulae

Six decimal places in a double might imply accuracy that isn't really
there, but you definitely don't want to try to cram it into a float and
expect a happy ending when you start your conversions.

A friend
who calibrated aeronautical navaids before he retired told me how they'd
set up a differential GPS transmitter at an airport and get 1-centimeter
accuracy while flying an approach.
(Worst-case WAAS is 20 feet vertically, less horizontally - competitive
with ILS.)

With DGPS, RTK, and post processing you can get the accuracy down to
millimeters.

I have a Amzfit Active 2 fitness watch that has a pretty good GPS. I'm
amused by the posters on reddit complaining that they took an out and back
hike and the two tracks aren't exactly on top of each other or doesn't
show them on the sidewalk. They expect miracles from a $100 consumer
device with a minimal patch antenna and also put a lot of trust in digital
maps.

I started using a GPS back in the '90s before Clinton took selective
availability off. Even then it was a revelation. I was used to dealing
with Forest Service maps of dubious accuracy and the notorious Forest
Service miles that were even worse. It was amusing that what I had
identified as Dead Skunk Point based on a crappy map and not knowing
exactly where I was really wasn't Dead Skunk Point.