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Best technique for storing custom map polygons in Android and ray casting

I'm fairly new to Android and trying to develop an app that identifies if a user's location is inside or outside of a given region within a state. My approach is to take the user's latlng and use ray casting to identify which region they are inside (they must be inside 1). My regions are best equated to state park lines, but Google does not have these in Google Maps (and they're too irregular for geofencing). As such, I created customer polygons. I'm not struggling with the code, but struggling with the best way to handle data.
How should I store and access the polygon data for ray casting? I was taking the approach of storing the polygons in an XML file but I'm worried about the time and processing power it may take to parse the XML and run a ray casting across up to 30 polygons in a given state. My polygons are complex enough that the XML file for one state is upwards of 4MB. My polygons only need to be read, not written, as they'll come with the app.

I think that your best option is to store your polygons is a geographic database. The best solution I've found so far to do so is SpatiaLite which is build over SQLite and works really well on Android.
Using this approach you will store you polygons in the database and query what polygons intersect with a given LatLng (Point). The query will look like this (not tested):
select * from polygons_table where st_intersects(Geometry, MakePoint(longitude, latitude, 4326));
Note that I use 4326 as the SRID because I assume that you will store your polygons in WGS84.
Here you can find the SpatiaLite 3.0.0-BETA SQL functions reference list.

Tracking GPS points and finding their nearest neighbours?

I have a list of 1 million (slowly) moving points on the globe (stored as latitude and longitude). Every now and then, each point requests a list of the 100 nearest other points (with a configurable max range, if that helps).
Unfortunately, SELECT * SORT BY compute_geodetic_distance() LIMIT 100 is too slow to be done by each point over and over again. So my question: how should I handle this efficiently? Are there better algorithms/datastructures/... known for this? Or is this the only way and should I look into distributing server load?
(Note: this is for an Android app and the points are users, so in case I'm missing an android-specific solution, feel free to say so!)

For your task geo spatial databases have been invented.
There is Oracle Spatial (expensive) and PostGres (free).
These databases store your millions points in a geographical index, a quad tree (Oracle).
Such a query needs nearly no time.
Some people, like me prefer to leave the database away and build up the quadtree themselfs.
The operations search and insert are easy to implement. Update/delete can be more complex.(Cheapest related to implementation effort, is to build up a new quadtree evry minute)
Using a quadtree you can perform hundreds or thousansds of such nearest 100 points within a second.

Architecturally I would arrange for each "point" to phone home to a server with their location when it changes more than a certain amount. On the server you can do the heavy lifting of calculating the distance between the point that moved and each of the other points, and for each of the other points updating their list of the 100 closest points if required. You can then push changes to a point's closest 100 list as they happen (trivial if you are using App Engine, Android push is supported).
This reduces the amount of work involved to an absolute minimum:
Only report a location change when a point moves far enough
Only recalculate distances when a report is received
Don't rebuild the closest 100 list for a point every time, build the list once, then work out if a point that has moved is going to be added or removed from every other point's list.
Only notify a point of changes to its top 100 list to preserve bandwidth.
There are algorithms that you can use to make this super-efficient, and the problem has a fork/join feel to it as well, allowing you to throw horsepower at the problem.

You have to divide the earth into zones and then use an interior point algorithm to figure out what zones the phone is in. Each possible subset of zones will uniquely determine the 100 closest nodes to a fair approximation. You can get an exact set of 100 nodes by checking distance one by one against the candidate nodes, which (once again) are determined by the subset of zones.

Instead of r-tree or a quadtree, I.e spatial index you can also use a quadkey and a monster curve. This curve reduce the dimension and completetly fills the space. You can download my php class hilbert curve from phpclasses.org. You can use a simple varchar column for the quadkey and search the levels from left to right. A good explanation is from Microsoft Bing maps quadkey website.

Save GPS Coordinates to file/DB/xml?

i'm trying my first android app and it should track my route with gps coorinates.
The app also has a five textboxes, each with about 30 chars user can type in.
Coordinates should be saved all 30/60 seconds, is this enough?
Or is it possible to save it with 10 seconds and what's the right way to save it?
Thought about reading xml from url, but I think it could be more data in future.
What could be a good way to store it on sd as XML or normal file locally, which I can parse from client-pc to retrieve coordinates?
Thanks for your time.
Best Regards

You should only save a position if it's far enough away from the previous position. That way you'll have way less data without losing any information (in other words - it doesn't help to save the same position every 10 seconds).
In my sports tracker app, I save the data in a database table (latitude, longitude, timestamp ... basically all you get in the Location object).

XML would work but the performance would dramatically decrease as the amount of data in your file increases. I had a similar project a year ago and I used a SQLite database.
The period you want to use depends on your needs, getting a location every 10 secs might be a lot, you might want to adapt the period to the speed or the area (city or highway). You can also rely on 3g (network instead of GPS) to get accurate, quicker and cheaper (in terms of battery) location fixes in dense areas (cities)

Consider the speed of the host, and the accuracy required when reconstructing the path. If you're walking, a sample every 30 seconds might be fine, but if you're in a car, you might want to sample faster. Also, I'd suggest XML, and I'd recommend looking up the GPX format, that would give you portability as well, because other programs will understand it and allow import/export.

getting multiple marker coordinates in google map android

Hi to all the members of this great community!
This is my first question so forgive me for possible mistakes. I hope that from this day on i can be helpful for some of you as hopefully you will be for me.Getting to the question:
I am building an android app whose purpose is to search for nearest fuel-points and nearest care-repair-centers. I am very new to android and thx to the numerous posts about android in here I have managed to reach the point where i have build the map and animate it to my current location while updating my location.
Now i have to add the markers of the points of interest. Since they are at least 10 (I will add them only for demonstration purposes) i think it's not wise to add them through 10+ repetitive calls to itemizedOverlay.addOverlayItem(). My idea was to save them in a file in the format ( " latitude " , "longitude" , simple_description_title , other info ) and than in some way import the first 2 fields for the geopoint and the 3rd for the title.
I will use than the 4th later for some type of tooltip text (for example tel_number).
Do you think this is a good approach? And how can I implement the file reading(if) in the code that extends ItemizeOverlay().
I didn't post the code until this point since it's irrelevant.

Welcome to SO, let's jump right into your problem/question.
1.) Since you are only adding 10 points of interest it won't matter if you just call itemizedOverlay.addOverlayItem() for all 10 because the trick is to call itemizedOverlay.populate() only after you have added all the overlayItems using itemizedOverlay.addOverlayItem(), this way you don't compromise on performance.
2.) Now, once again, since you are only doing a demonstration I would advise you to simply hard-code all the 10 overlays with their respective geolocations into the Android code itself. This way you WON'T have to worry about reading data. Also, using a txt file to store data isn't the best option both performance and convenience wise. This is what databases exist for.
3.) If, and when you do this in the future, you do need to use some dynamic data to populate your markers with, then I'd STRONGLY advise you to use either
SQLite: The embedded database that Android offers, it's great for storing small bits of information that's required for your application such as description title, other info, the latitude, longitude, however, if you have some sort of a connection based application where you need to update globally accessible data every once in a while I'd advise you to use the next option,
MySQL: This is an online database that you have to interface with using a server and PHP. The advantage of using an online database is that you can now share information between different users (friends, contacts, followers etc.) by reading and writing to and from the database.

Compare values with values from a file?

What would be the best way to do this?
I have an application that gets two values about each and every 10th second (when user touches the screen). From this i get two values, latitude and longitude of a sphere object that the user has touched.
Now I would like to compare thoose values to values from a file, with the real latitude longitude of a location and then compare thoose values and se how far away the user was.
My file will be built up with two values and one key (location) in each index.
What would be the best way to do this, would it be to read the whole file in the beggining with a bufferedInputStreamReader and store thoose in a
HashMap<String, List<Float>>
or would i be better of using some kind of database structure like SqlLite?
Since Im doing this on a mobile platform performance is quite important and that's mainly why i ask this question.

Depending on the size of the data you need to compare against, you could either look up each time against a database (slower) or do a binary search in memory (faster).
If you store in a HashMap (for the in memory method), then you will need to sort and implement a binary search for maximum speed. Otherwise you will be searching linearly (iterating) throughout the collection of values (that might be acceptable to you).
I would say if you have a few thousands entries, then do it in memory, if you have more then go down the database route.