gps interview questions
Top gps frequently asked interview questions
I'm working with GPS data, getting values every second and displaying current position on a map. The problem is that sometimes (specially when accuracy is low) the values vary a lot, making the current position to "jump" between distant points in the map.
I was wondering about some easy enough method to avoid this. As a first idea, I thought about discarding values with accuracy beyond certain threshold, but I guess there are some other better ways to do. What's the usual way programs perform this?
Source: (StackOverflow)
How can I mock my location on a physical device (Nexus One)? I know you can do this with the emulator in the Emulator Control panel, but this doesn't work for a physical device.
Source: (StackOverflow)
How can I check the current status of the GPS receiver? I already checked the LocationListener onStatusChanged method but somehow it seems that is not working, or just the wrong possibility.
Basically I just need to know if the GPS icon at the top of the screen is blinking (no actual fix) or solid (fix is available).
Source: (StackOverflow)
I know that the question about turning on/off GPS programatically on android has been discussed many times, and the answer is always the same:
"You can't for security/privacy reasons, you have to forward to location preferences screen and let the user enable/disable it."
I understand that, however I recently bought Tasker from the market and, among many other things that you can accomplish with it, you can set rules to auto-enable GPS on entering pre-determined applications and disable it on exit (see here for the tutorial on how to do it, and it just works!) and this app can't be signed with the firmware signing key as it works on many android versions and different devices and you don't even need to be rooted.
I would like to do this in my app. Of course, I don't want to blow up the users privacy, so I would first ask the user if he wants to turn it on automatically with the typical "remember my decision" checkbox and if he answers yes, enable it.
Does anybody have any idea or clue on how Tasker achieves this?
Source: (StackOverflow)
In a GPS app that allows the user to display a list of complex location points that we call tracks on various different types of map, each track can consist of between 2k to 10k of location points. The tracks are copiously clipped, pruned and path-simplified when they are rendered on non-Google map types. This is to keep memory usage down and performance up. We typically only wind up submitting far less than a thousand (aggregate) transformed location points to the OpenGL pipeline, even in the worst cases.
In integrating the Google Maps SDK for iOS, we initially attempted to continue to leverage our own OpenGL track rendering system, but ran into issues with conflicting OpenGL context usage (rendering worked, but we couldn't get GMSMapView to and our own internal OpenGL resources to both release without someone touching deleted memory).
So we are trying to leverage the GMSPolyline constructs and just let the Google SDK do the track rendering, but we've run into major memory usage issues, and are looking for guidance in working around them.
Using Xcode Instruments, we've monitored memory usage when creating about 25 poly lines with about 23k location points total (not each). Over the course of poly line creation, app memory usage grows from about 14 MB to about 172 MB, a net peak of about 158 MB. Shortly after all the poly lines are created, memory usage finally drops back down to around 19 MB and seems stable, for a cumulative net of around 5 MB, so it seems each location point requires around 220 bytes (5 MB / 23k points) to store.
What hurts us is the peak memory usage. While our laboratory test only used 23k location points, in the real world there are often many more, and iOS seems to jettison our application after Google Maps has consumed around 450 MB on an iPhone 5 (whereas our internal poly line rendering system peaks at around 12 MB for the same test case).
Clearly the GMSPolyLine construct is not intended for the heavy weight usage that we require.
We tried wrapping some of the poly line creation loops with separate autorelease pools, and then draining those at appropriate points, but this has no impact on memory use. The peak memory use after the poly lines are created and control is returned to the main run loop didn't change at all. Later it became clear why; the Google Map system isn't releasing resources until the first DisplayLink callback after the poly lines are created.
Our next effort will be to manually throttle the amount of data we're pushing at GMSPolyline, probably using our own bounds testing, clipping, pruning & minimization, rather than relying on Google Maps to do this efficiently.
The drawback here is that it will mean many more GMSPolyline objects will be allocated and deallocated, potentially while the user is panning/zooming around the map. Each of these objects will have far fewer location points, yet still, we're concerned about unforeseen consequences of this approach, hidden overhead of many GMSPolyline allocs and deallocate.
So the question is, what is the best approach for dealing with this situation, and can someone from Google shed some light on any GMSPolyline best practices, upper bounds, bottlenecks, etc. ?
Source: (StackOverflow)
hard-coding the setZoom() within onCreate() feels very antiquated and I'd like to enhance the user experience by initially having the MapView set the zoom until all GeoPoints / OverlayItems are visible on the map.
How can this be done auto-magically?
Source: (StackOverflow)
I've recently known about a gps.conf file in the /system/etc/ directory.
Seems that tweaking NTP_SERVER values to NTP servers nearer to the usual location improves TTFF.
Reading the source code in the LocationProvider class, seems that at boot, time is retrieved from NTP server and "injected" in the calculations.
AFAIK each GPS sat has a very accurate atomic clock, and every one in the constellation is synchronized to the so called "GPS time". Once the receiver has got 4 or more satellites, it solves (by some method) an equation where there are four unknowns: x,y,z,b; where (x,y,z) is the receiver location, and b is the time difference between the receiver internal clock and the (correct) GPS time. Once it has a a fix, the receiver clock is synced with the correct time. (Please correct me if I'm wrong).
So far, I've some questions regarding the way NTP time injection works:
- GPS Time is roughly TAI (International Atomic Time) plus an offset. Those two times do not depend on the Earth rotation, however UTC does. Given that NTP servers return UTC time, it is possible to infer GPS Time from UTC time?
- How does retrieving NTP time from a nearer server improve the "quality" of the GPS time approximation?
- Assuming that we have an initial GPS Time value (inferred from NTP time somehow), what is the injection about? Is this time value taken as correct to solve the equation with only x,y,z as unknowns? If so, then the first fix is also just an approximation, isn't it?
- How does a higher quality initial approximation for GPS time improve TTFF? Is it because with a lower quality NTP time the first fixes are considered not acceptable and discarded?
- Does having an approximate initial position help retrieving the next correct fix (like listening only to a subset of sats)?
Source: (StackOverflow)
I'm stumped regarding how to implement a "personal compass", ie a compass that points to a specific bearing instead of the standard "north pole"... unfortunatly, my current attempt has come out wrong (doesn't point at the given bearing). It's also hooked up with the accelerator to be able to dynamically adjust itself based on which way the user is turning.
Here's my current attempt at it (the onSensorChanged()-method that updates the arrow):
public void onSensorChanged( SensorEvent event ) {
// If we don't have a Location, we break out
if ( LocationObj == null ) return;
float azimuth = event.values[0];
float baseAzimuth = azimuth;
GeomagneticField geoField = new GeomagneticField( Double
.valueOf( LocationObj.getLatitude() ).floatValue(), Double
.valueOf( LocationObj.getLongitude() ).floatValue(),
Double.valueOf( LocationObj.getAltitude() ).floatValue(),
System.currentTimeMillis() );
azimuth += geoField.getDeclination(); // converts magnetic north into true north
//Correct the azimuth
azimuth = azimuth % 360;
//This is where we choose to point it
float direction = azimuth + LocationObj.bearingTo( destinationObj );
rotateImageView( arrow, R.drawable.arrow, direction );
//Set the field
if( baseAzimuth > 0 && baseAzimuth < 45 ) fieldBearing.setText("S");
else if( baseAzimuth >= 45 && baseAzimuth < 90 ) fieldBearing.setText("SW");
else if( baseAzimuth > 0 && baseAzimuth < 135 ) fieldBearing.setText("W");
else if( baseAzimuth > 0 && baseAzimuth < 180 ) fieldBearing.setText("NW");
else if( baseAzimuth > 0 && baseAzimuth < 225 ) fieldBearing.setText("N");
else if( baseAzimuth > 0 && baseAzimuth < 270 ) fieldBearing.setText("NE");
else if( baseAzimuth > 0 && baseAzimuth < 315 ) fieldBearing.setText("E");
else if( baseAzimuth > 0 && baseAzimuth < 360 ) fieldBearing.setText("SE");
else fieldBearing.setText("?");
}
And here's the method that rotates the ImageView (rotateImageView()):
private void rotateImageView( ImageView imageView, int drawable, float rotate ) {
// Decode the drawable into a bitmap
Bitmap bitmapOrg = BitmapFactory.decodeResource( getResources(),
drawable );
// Get the width/height of the drawable
DisplayMetrics dm = new DisplayMetrics(); getWindowManager().getDefaultDisplay().getMetrics(dm);
int width = bitmapOrg.getWidth(), height = bitmapOrg.getHeight();
// Initialize a new Matrix
Matrix matrix = new Matrix();
// Decide on how much to rotate
rotate = rotate % 360;
// Actually rotate the image
matrix.postRotate( rotate, width, height );
// recreate the new Bitmap via a couple conditions
Bitmap rotatedBitmap = Bitmap.createBitmap( bitmapOrg, 0, 0, width, height, matrix, true );
//BitmapDrawable bmd = new BitmapDrawable( rotatedBitmap );
//imageView.setImageBitmap( rotatedBitmap );
imageView.setImageDrawable(new BitmapDrawable(getResources(), rotatedBitmap));
imageView.setScaleType( ScaleType.CENTER );
}
Any help would be much appreciated, as I don't quite know how to proceed. The "readings" I'm getting while trying it out is somewhat inaccurate and points in the wrong direction. Am I doing something really off, or did I just have a really bad test-run?
Source: (StackOverflow)
i need to monitor user's locations using a background service , and then load them and show the path to the user.
using an activity , it was quite easy to get gps locations , but when i got to do it via a service , i came into a problem as it seems to only work for looper threads (or something like that) .
as i've searched the internet for a solution, i've found out that many people got the same problem, but i couldn't find a working solution . some people say that you need to use prepare->loop->quit , and some say you have to use a handlerThread , but still, i can't find out how to do such things in a proper way .
can anyone please help me ? maybe provide a minimalistic solution?
Source: (StackOverflow)
I would like to extract the GPS EXIF tag from pictures using php.
I'm using the exif_read_data() that returns a array of all tags + data :
GPS.GPSLatitudeRef: N
GPS.GPSLatitude:Array ( [0] => 46/1 [1] => 5403/100 [2] => 0/1 )
GPS.GPSLongitudeRef: E
GPS.GPSLongitude:Array ( [0] => 7/1 [1] => 880/100 [2] => 0/1 )
GPS.GPSAltitudeRef:
GPS.GPSAltitude: 634/1
I don't know how to interpret 46/1 5403/100 and 0/1 ? 46 might be 46° but what about the rest especially 0/1 ?
angle/1 5403/100 0/1
What is this structure about ?
How to convert them to "standard" ones (like 46°56′48″N 7°26′39″E from wikipedia) ? I would like to pass thoses coordinates to the google maps api to display the pictures positions on a map !
Source: (StackOverflow)
This question already has an answer here:
I'm having troubles of getting my current position coordinates using the NETWORK provider of android location system.
Already read a lot of tutorials and implemented 4 or 5 existing classes to my project and all of them are giving me the last coordinates but not the current ones.
I'm pretty sure that the problem is something fundamental that I am missing but I am not able to understand what exactly it is.
Codes i'm using now:
This is my main Activity
package com.example.locationtests;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.widget.TextView;
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
GPSTracker mGPS = new GPSTracker(this);
TextView text = (TextView) findViewById(R.id.texts);
if(mGPS.canGetLocation ){
mGPS.getLocation();
text.setText("Lat"+mGPS.getLatitude()+"Lon"+mGPS.getLongitude());
}else{
text.setText("Unabletofind");
System.out.println("Unable");
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
}
This is the class im using for Tracking:
package com.example.locationtests;
import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Bundle;
import android.provider.Settings;
import android.util.Log;
public final class GPSTracker implements LocationListener {
private final Context mContext;
// flag for GPS status
public boolean isGPSEnabled = false;
// flag for network status
boolean isNetworkEnabled = false;
// flag for GPS status
boolean canGetLocation = false;
Location location; // location
double latitude; // latitude
double longitude; // longitude
// The minimum distance to change Updates in meters
private static final long MIN_DISTANCE_CHANGE_FOR_UPDATES = 1; // 10 meters
// The minimum time between updates in milliseconds
private static final long MIN_TIME_BW_UPDATES = 1; // 1 minute
// Declaring a Location Manager
protected LocationManager locationManager;
public GPSTracker(Context context) {
this.mContext = context;
getLocation();
}
/**
* Function to get the user's current location
*
* @return
*/
public Location getLocation() {
try {
locationManager = (LocationManager) mContext
.getSystemService(Context.LOCATION_SERVICE);
// getting GPS status
isGPSEnabled = locationManager
.isProviderEnabled(LocationManager.GPS_PROVIDER);
Log.v("isGPSEnabled", "=" + isGPSEnabled);
// getting network status
isNetworkEnabled = locationManager
.isProviderEnabled(LocationManager.NETWORK_PROVIDER);
Log.v("isNetworkEnabled", "=" + isNetworkEnabled);
if (isGPSEnabled == false && isNetworkEnabled == false) {
// no network provider is enabled
} else {
this.canGetLocation = true;
if (isNetworkEnabled) {
location=null;
locationManager.requestLocationUpdates(
LocationManager.NETWORK_PROVIDER,
MIN_TIME_BW_UPDATES,
MIN_DISTANCE_CHANGE_FOR_UPDATES, this);
Log.d("Network", "Network");
if (locationManager != null) {
location = locationManager
.getLastKnownLocation(LocationManager.NETWORK_PROVIDER);
if (location != null) {
latitude = location.getLatitude();
longitude = location.getLongitude();
}
}
}
// if GPS Enabled get lat/long using GPS Services
if (isGPSEnabled) {
location=null;
if (location == null) {
locationManager.requestLocationUpdates(
LocationManager.GPS_PROVIDER,
MIN_TIME_BW_UPDATES,
MIN_DISTANCE_CHANGE_FOR_UPDATES, this);
Log.d("GPS Enabled", "GPS Enabled");
if (locationManager != null) {
location = locationManager
.getLastKnownLocation(LocationManager.GPS_PROVIDER);
if (location != null) {
latitude = location.getLatitude();
longitude = location.getLongitude();
}
}
}
}
}
} catch (Exception e) {
e.printStackTrace();
}
return location;
}
/**
* Stop using GPS listener Calling this function will stop using GPS in your
* app
* */
public void stopUsingGPS() {
if (locationManager != null) {
locationManager.removeUpdates(GPSTracker.this);
}
}
/**
* Function to get latitude
* */
public double getLatitude() {
if (location != null) {
latitude = location.getLatitude();
}
// return latitude
return latitude;
}
/**
* Function to get longitude
* */
public double getLongitude() {
if (location != null) {
longitude = location.getLongitude();
}
// return longitude
return longitude;
}
/**
* Function to check GPS/wifi enabled
*
* @return boolean
* */
public boolean canGetLocation() {
return this.canGetLocation;
}
/**
* Function to show settings alert dialog On pressing Settings button will
* lauch Settings Options
* */
public void showSettingsAlert() {
AlertDialog.Builder alertDialog = new AlertDialog.Builder(mContext);
// Setting Dialog Title
alertDialog.setTitle("GPS is settings");
// Setting Dialog Message
alertDialog
.setMessage("GPS is not enabled. Do you want to go to settings menu?");
// On pressing Settings button
alertDialog.setPositiveButton("Settings",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
Intent intent = new Intent(
Settings.ACTION_LOCATION_SOURCE_SETTINGS);
mContext.startActivity(intent);
}
});
// on pressing cancel button
alertDialog.setNegativeButton("Cancel",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
dialog.cancel();
}
});
// Showing Alert Message
alertDialog.show();
}
@Override
public void onLocationChanged(Location location) {
}
@Override
public void onProviderDisabled(String provider) {
}
@Override
public void onProviderEnabled(String provider) {
}
@Override
public void onStatusChanged(String provider, int status, Bundle extras) {
}
}
This is my AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.locationtests"
android:versionCode="1"
android:versionName="1.0" >
<uses-permission android:name="android.permission.CAMERA" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
<uses-permission android:name="android.permission.INTERNET"></uses-permission>
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.VIBRATE" />
<uses-feature android:name="android.hardware.camera" />
<uses-feature android:name="android.hardware.location" android:required="true" />
<uses-feature android:name="android.hardware.location.gps" android:required="false" />
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="17" />
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name="com.example.locationtests.MainActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
Acctualy GPS possitioning works fine, but NETWORK positioning dosn't.
When Ive turned devices GPS on cordinates keep changing while im moving, but dosnt happence same when I have turn it off and relay on the NETWORK_PROVIDER
Source: (StackOverflow)
Do Android Devices use the network time protocol (NTP) to synchronize the time?
In my Device-Settings I see a checkbox with the following text "synchronize with network", but I don't know if they are using NTP.
I need this for my Bachelor Thesis for which I use GPS. To get a accurate GPS-signal the receiver should have a exact clock.
I found this blog-entry on the web, but I'm not sure if he tell the truth: Speeding up NTP, GPS Lock in Android
Source: (StackOverflow)