which gps app can do this?

Helge Hafting helge.hafting at hist.no
Mon Jun 29 13:49:43 CEST 2009

lanzo wrote:
> Hi!
> I'd like to be using my FR on my little boat as marine GPS. I was curious
> if, in your opinion, it could be possible to constantly show  the distance
> between me and the nearest point on the coast line. This would be important
> because in my country (and i guess everywhere) there are rules about the
> little boats distances from the coast and I cannot overcome 3 nautical
> miles.

If this is a safety feture, take care to use map data that is 
sufficiently accurate for your use. Also, the freerunner is not built to 
withstand salt water, so make sure it won't get wet.

> I know it should be possible to show the distances between my present
> position and any given point, but what about something always displaying the
> distances between my position and the nearest point on the coast?
I don't know any such app, the use seems a bit special to me. So you may 
have to use an existing app, and add code to it for your use.

An approach using navit - requires programming:

I guess navit is a good choice. It has maps in vector format, including 
coastline. So you could add some code that periodically checks
that at least some coast points are within 3 nautical miles.

An approach using tangogps - no programming at all:

Tangogps performs better than navit on the freerunner, it is faster.
Tangogps uses a map made from png tiles. So there is no way to find the 
distance to the coast. However, you can make your own map (based on 
openstreetmap tiles) that includes the 3 mile border. You could have a 
line in the water, or color the forbidden regions differently. Then, you 
simply look at the map display now and then to check that you are within 
the allowed zone.

The absolutely simplest way is to edit the png tiles with an image 
editor, and just draw the border line/area yourself. Openstreetmap has 
the information you need about scale. (How big regions the tiles cover.)
This way require no programming at all, you just draw the (rough) limits 
onto the existing map. The tiles are small - you might have to edit lots 
of them depending on what zoom level and how big an area you need maps for.

Better approach for tangogps - requires some software and configuration:

A more elegant way is to render the tiles for your region yourself. 
openstreetmap.org has information on how you download software to do 
this. It is some one-time work downloading and installing the software.

After that, you need to modify the rules for rendering, so that allowed 
and disallowed water is rendered with different color. (Or a borderline 
3 miles outside the coast). Then, run the software so it renders maps 
for your region. This approach has the advantage that you can update 
with new data from openstreetmap now and then, and have your map improve 
with time. Perhaps you won't need to do actual programming, but you will 
have to work with rendering rules (and install some software).

> is there maybe any more in-topic forum where i can ask this?
Your question is definitely on-topic, because you are using a 
freerunner. Don't worry about that. :-)

For detailed help, note that navit has a mailing list, and openstreetmap 
has several mailing lists/forums where they can help you with rendering 
questions and other tech stuff.

Helge Hafting

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