[GIS] How to decode GPS coordinates from Android device XML file

androidcoordinatesgps

I’d like to decode the location coordinates from the XML file from my Android phone (SGS2). Any solution how to do it, how to convert them to usable coordinates (longitude/latitude)?
Looks like it's base64 encoded. from there, no idea.

<string name="lastPosition">wLphlm9S56tDr+Ruft1URA4iubjA111HfIdVuhw3cC8=</string>

Full XML example:

<map>
 <string name="lastPosition">1j+wY60EYdawR3UTnoSsbAcGWRDIlNTCd0j3zXZO1D4=</string>
 <int name="deviceTag" value="1118448321"/>
 <long name="locationReportingIntentTimstamp" value="1363120660401"/>
 <float name="lastAccuracy" value="759.0"/>
 <boolean name="locationHistoryEnabled" value="true"/>
 <long name="lastTimestamp" value="1363121442835"/>
 <long name="lastIntentProcessedTimestamp" value="1363121443003"/>
</map>

Best Answer

This looks like an encoded polyline to me - for the algorithm to encode/decode, see:

https://developers.google.com/maps/documentation/utilities/polylinealgorithm

Google also has an online tool that can encode/decode:

https://developers.google.com/maps/documentation/utilities/polylineutility

As you noted decoding your polyline seems to result in locations off the coast of Africa:

enter image description here

Judging by the lastTimestamp in the XML file (and assuming this is UTC in milliseconds), these locations were recorded back in 2013 (Tue, 12 Mar 2013 20:50:42 GMT to be exact). So, this data may not be valid tracking data.

I know manufacturers have gotten in trouble in the past over logging device locations to system files as part of the device development process, and then they forget to remove/purge these files before the software is launched to production. My guess is that this file is a leftover debug file from Samsung's internal software testing with the Galaxy S2.

Related Solutions

[GIS] Determine current address location in android mobile app

Yes, using the server-side Geocoding API without displaying the results on a Google Map violates Google's Maps API Terms-of-Service.

From Section 10.1.1(h):

No Use of Content without a Google Map. You must not use or display the Content without a corresponding Google map, unless you are explicitly permitted to do so in the Maps APIs Documentation. In any event, you must not use or display the Content on or in conjunction with a non-Google map. For example, you must not use geocodes obtained through the Service in conjunction with a non-Google map. As another example, you must not display Street View imagery alongside a non-Google map, but you may display Street View imagery without a corresponding Google map because the Maps APIs Documentation explicitly permits you to do so.

One alternative on Android is the Android Geocoder, which doesn't have that terms-of-service restriction. The Geocoder.getFromLocation() method takes in a latitude and longitude and returns a list of possible address matches. Another benefit of the Android Geocoder is that there are no usage limits. I created a sample Android Geocoder app on Github that you can feel free to use (licensed under Apache 2.0) (an aside - I created the app originally to demo a bug on Android that's since been fixed).

There are also other alternatives. You might want to check out the OpenTripPlanner for Android app on Github that our group has worked on - we support multiple geocoders (including the MapQuest Nominatim API), as discussed in the OTPGeocoding wiki. Here's the code for the OTPGeocoding task, and the utility method that actually makes the calls.

[GIS] Finding correct X Y values from two pairs of X Y map coordinates

You need to loop over both dimensions x and y and compare the distance of every lon-lat pair in the x-y grids from your desired lon-lat point.

You seem to have indicated that in addition to the projection definition there are also supplemental 2D lon and lat arrays in the file, so you can just try to work with those rather than code up the map projection transformation equations.

So writing this very crudely in Java, it might look something like:

double ixmin = -1; // x index at closest grid point
double iymin = -1; // y index at closest grid point
double mindist = Double.INFINITY;

for (int iy = 0; iy < ylength; ++iy) {
    for (int ix = 0; ix < xlength; ++ix) {
        double dlon = mylon - lon(iy,ix);
        double dlat = mylat - lat(iy,ix);
        double dist = Math.sqrt (dlon * dlon + dlat * dlat);

        if (dist < mindist) {
            mindist = dist;
            iymin = iy;
            ixmin = ix;
        }
    }
}

System.out.println ("Closest grid point is at iy=" + iy + ", ix=" + ix);

Note that the above does not account for possible differences in handling the lon range. Some files will specify longitudes running from (0,360), others (-180,180). So you need to make sure that you don't have a dlon value whose abs value is over 360.

Best Answer

Related Solutions

[GIS] Determine current address location in android mobile app

[GIS] Finding correct X Y values from two pairs of X Y map coordinates

Related Question