When you convert from degrees to meters, choose your projection carefully.
Different projections have different properties, e.g. doing better at
preserving distance, angles, etc. Choose the wrong projection, e.g. Mercator,
and you could have some wild discrepancies with reality.
On Aug 30, 2013, at 3:44 PM, André Salvati <[email protected]> wrote:
> Srs,
>
> just a question. As I'm dealing with GPS data, I'm getting latitude and
> longitude in degrees, but as you know I can't build JTS shapes with
> "degrees". So, I'm looking for an API to convert this position in meters on
> the Earth's surface.
>
> I've tried GeoTools, but it doesn't goes well with my servers (Google App
> Engine).
>
> Would you recommend any other?
>
> Thanks
>
>
> 2013/8/26 André Fernando Salvati <[email protected]>
>
> Ok Martin,
>
> I'm going to do some tests and share the results.
>
> Thank you all for your comments and ideas.
>
>
> 2013/8/26 Martin Davis <[email protected]>
> This is a very interesting question. Stefan and Michael have provided some
> useful comments. Here's a little more information about the issue, focussing
> on what's in JTS today, and what could be added to improve the story.
>
> As you point out, there are two general approaches to this computation:
>
> 1) buffer the LIneString by the query distance and compute "contains" (or
> actually "containsProperly", which is a simpler predicate)
> 2) Compute the isWithinDistance predicate on the LineString
>
> It's hard to say which is faster *now* in JTS, and even harder to say which
> is theoretically the fastest. You'll really have to try both and see.
> Here's what's in JTS now:
>
> For #1, you should use PreparedGeometry to preprocess the buffer polygon. My
> guess is that the cost of computing the initial buffer is totally dominated
> by the query time. Then use the "cointainsProperly" predicate. This will
> utilized indexing to optimize the Point-In-Polygon computation. It is
> thread-safe now in trunk, so you may want to build from trunk and use that
> code.
>
> For #2, the current JTS isWithinDistance is not fully optimized for large
> LineStrings, since it uses a brute-force approach. Still, if the input
> LineString has a fairly low number of vertices it may still be faster than
> #1. As Michael pointed out, this can be improve by indexing the LineString
> segments using an STRtree and using the nearestNeighbour method to do a fast
> NN query. The intention is to implement this in JTS as new methods on the
> PreparedGeometry class - most of the code is done, but it still needs to be
> packaged for production.
>
> It would be very interesting to hear which of these methods turns out to be
> the fasted in your case.
>
>
>
> On Mon, Aug 26, 2013 at 10:21 AM, André Salvati <[email protected]>
> wrote:
> Hi,
>
> I'm a beginner with JTS and this is my first post here.
>
> We've been working with geofences for a vehicle tracking app and I would like
> to know what solution could give me a lower CPU consumption.
>
> 1) Calculate and store a Buffered Area (100 meters) from a LineString object.
> Run 4 million times if my vehicle positions are inside or not that
> precalculated with contains() method.
>
> 2) Run 4 million times isWithinDistance() method from LineString for each
> position.
>
> Are there another options?
>
> Thanks.
>
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