Important in judging the risk of collisions, but it is a much simpler

proposition if you don't have to stay on a road.

There is some nautical freeware called SeaClear II available at sping.com

for anyone that would like to see what one implementation looks like.

You'll have to provide an NMEA location feed to simulate your own movement

and find an online AIS feed to simulate other traffic. I think there is one

available for San Francisco Bay but don't have the link handy.

I run both SeaClear and APRSIS32 on my car laptop sometimes as we have OSM

and nautical chart coverage of the roads and bridges I drive. Kinda funny

to be warned that your Explorer is in danger of collision with an oil tanker

that's 100 feet below the roadbed.

I'd be surprised if anyone had implemented a similar solution for the road

atlas based topology used in automotive GPS, but for rural and freeway

driving, the simple marine CPA may have some utility and not be too hard to

implement.

-----Original Message-----

From: aprsisce@yahoogroups.com [mailto:aprsisce@yahoogroups.com] On Behalf

Of James Ewen

Sent: Thursday, June 28, 2012 8:48 PM

To: aprsisce@yahoogroups.com

Subject: Re: [aprsisce] feature request mid point

On Thu, Jun 28, 2012 at 5:58 PM, Steve Daniels

<steve@...> wrote:

> It's a help to make a decision, not a be all and end all to things.

Yup, but I was just playing the opposite side of the fence... Driving on a

>

> I don't blindly follow directions and neither do you I expect James.

> You get help from the tech.

straight road towards each other makes the job of figuring out a mid-point

fairly easy.

Without any routing information, all you can do is draw a straight line

between the two points. You could take the apparent closing speed of each

unit and try to come up with a mid-point based on those numbers, but how

good would that estimate be, and would it be any better than what you could

do just looking at the map?

Around here it would probably work out most of the time for two people

driving towards each other because our roads are fairly straight.

Take this example though... I'm driving northbound on road A, and you are

driving westbound on road B. These roads are perfectly straight, and

intersect perpendicular to each other at point C. We are both travelling 60

km/h, and are 30 km from point C. Simple observation would suggest that the

most useful meeting point would be at point C, and we would meet there in

1/2 an hour.

If you calculated a straight line between us, the meeting point would be

halfway along the hypotenuse 21 km from each of us. The velocity made good

towards each other along that hypotenuse would be 42 km/h, even though we

aren't moving in that direction, and that VMG would decrease as the angle

towards the location increased away from our direction of travel,

effectively making longer and longer to get to the meeting point until we

end up moving away from the mid point. Of course with the calculations

happening as we drive towards each other, the mid point will move at a 45

degree angle towards point C, at which time we will have arrived at the

meeting place that we already knew would be the best location for both of

us.

As the angle of intersection between the two roads decreases, the apparent

midpoint would get closer and closer even though point C would still be the

best probable meeting point without taking either of us out of our way.

Hopefully this example shows why simple mathematical calculations might not

be all that helpful with no knowledge of the underlying topology of the

roads.

It's a similar problem when I search for a POI on the GPS. I tell the GPS to

look for a gas station for me. It tells me the closest one is 2 km away.

Great, I tell it to take me there, and the GPS starts telling me to travel

away from my desired destination because the gas station is in the opposite

direction. However, the next closest gas station that was 3 km away is right

on my desired route, and is much more convenient for me, but since the GPS

has no idea where I intend to go (I'm sitting in a parking lot playing with

the GPS with no route selected), all it can do is suggest gas stations based

on proximity.

I'm not saying the concept is bad, but the amount of information needed to

make an informed decision based on all available criteria is very complex.

It might not seem like it, but when you break it down into steps, it can be

very complex. That grey goo between our ears does some pretty fancy

calculations for us some days.

--

James

VE6SRV

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