[MoNav Dev] Precision of UnsignedCoordinate::ToGPSCoordinate()

[Christoph Eckert]

Hi,

compared to Mappero, which provides lat and lon with 6 decimal digits, I get
only 4 digits for lat and 5 digits for lon from MoNav - which is a bit
inaccurate.

I have no clue whether this is caused by QGeoPositionInfoSource or my
own/MoNav's code:

const RoutingLogic::GPSInfo& gpsInfo = RoutingLogic::instance()->gpsInfo();

qDebug() << gpsInfo.position.x << gpsInfo.position.y;
qDebug() << gpsInfo.position.ToGPSCoordinate().latitude \
<< gpsInfo.position.ToGPSCoordinate().longitude;

results in something like

561900976 368759099
48.9973 8.39198
561900931 368759164
48.9973 8.39197
561900925 368759161
48.9973 8.39197
561900891 368759111
48.9973 8.39195
561900852 368759081
48.9973 8.39194
561900862 368759041
48.9973 8.39194

From coordinates.h:

/** converts to GPS coordinate */
GPSCoordinate ToGPSCoordinate() const
{
return ToProjectedCoordinate().ToGPSCoordinate();
}

/** converts to floating point coordinate */
ProjectedCoordinate ToProjectedCoordinate() const
{
ProjectedCoordinate tile;
if ( !IsValid() )
return tile;
tile.x = x;
tile.y = y;
tile.x /= ( 1u << 30 );
tile.y /= ( 1u << 30 );
return tile;
}


/** converts into a GPS position */
GPSCoordinate ToGPSCoordinate() const
{
GPSCoordinate gps;
if ( !IsValid() )
return gps;

gps.longitude = x * 360.0 - 180;
const double n = M_PI - 2.0 * M_PI * y;
gps.latitude = 180.0 / M_PI * atan( 0.5 * ( exp( n ) - exp( -n ) )
);
return gps;
}


Admittedly I do not exactly understand what the above calculations do. In case
someone savvy could shed a little light on them, it would be much appreciated.
In case it helped to remove one item from my TODO list, even better :) .

[Christian Vetter]

Hi,

Post by Christoph Eckert
Hi,
compared to Mappero, which provides lat and lon with 6 decimal digits, I get
only 4 digits for lat and 5 digits for lon from MoNav - which is a bit
inaccurate.
I have no clue whether this is caused by QGeoPositionInfoSource or my
const RoutingLogic::GPSInfo& gpsInfo = RoutingLogic::instance()->gpsInfo();
qDebug() << gpsInfo.position.x << gpsInfo.position.y;
qDebug() << gpsInfo.position.ToGPSCoordinate().latitude \
<< gpsInfo.position.ToGPSCoordinate().longitude;

The reduced accuracy is only caused by the output. qDebug outputs with
a default accuracy of 6 digits ( including the ones before the
decimal point ). So you have to increase the floating point accuracy
of whatever output functions you are using. I do believe that qDebug
cannot be modified to output with increased accuracy. However, you can
use QString().arg(...) (
http://doc.qt.nokia.com/latest/qstring.html#arg-20 ).

UnsignedCoordinate itself should be accurate to at least 4cm ( worst
accuracy on the equator ). The conversion from GPS to
ProjectedCoordinate transforms the coordinate into the Mercator
projection used by OSM, x in [0,1], y in [0,1]. This is beneficial
since most plugins use this projection. UnsignedCoordiante simple
stores x and y as unsigned int, stretching the range [0,1] to
[0,2^30].

Regards,

Christian Vetter

[Christoph Eckert]

Hi,

Post by Christian Vetter
The reduced accuracy is only caused by the output. qDebug outputs with
a default accuracy of 6 digits ( including the ones before the
decimal point ). So you have to increase the floating point accuracy
of whatever output functions you are using.

I've seen this in the tracklog, using QString::number(). Increasing its
precision to 6 digits only gave me additional 0's. Based on your hint, I'll
try anew. In case the problem persists, I'll come back with some more details.

BTW: I've added a first german translation to the uing branch. Except for some
minor hickups, it looks great on the N900 and the desktop so far.


Gn8,

ce

[Christoph Eckert]

Hi,

Post by Christian Vetter
The reduced accuracy is only caused by the output. qDebug outputs with
a default accuracy of 6 digits ( including the ones before the
decimal point ). So you have to increase the floating point accuracy
of whatever output functions you are using. I do believe that qDebug
cannot be modified to output with increased accuracy. However, you can
use QString().arg(...) (
http://doc.qt.nokia.com/latest/qstring.html#arg-20 ).

thanks a bunch for the pointer. Of course "it was all my fault" - the
precision obviously went down as I had the device on my desk…

Outdoors, either of the following provide the desired results:
double latitude = m_gpsInfoBuffer.at(i).position.ToGPSCoordinate().latitude;

std::cout.precision( 6 );
cout << latitude;

QString::number( latitude, 'f', 6 )

I apologize for the traffic, but at least we now have more accurate logging for
MoNav :) .

[Christian Vetter]

Hi,

Glad that's fixed. I already fell into that trap several times myself
when coding GPS loggers.

Regards,

Christian Vetter

Post by Christoph Eckert
Hi,

Post by Christian Vetter
The reduced accuracy is only caused by the output. qDebug outputs with
a default accuracy of 6 digits ( including  the ones before the
decimal point ).  So you have to increase the floating point accuracy
of whatever output functions you are using. I do believe that qDebug
cannot be modified to output with increased accuracy. However, you can
use QString().arg(...) (
http://doc.qt.nokia.com/latest/qstring.html#arg-20 ).

thanks a bunch for the pointer. Of course "it was all my fault" - the
precision obviously went down as I had the device on my desk…
double latitude = m_gpsInfoBuffer.at(i).position.ToGPSCoordinate().latitude;
std::cout.precision( 6 );
cout << latitude;
QString::number( latitude, 'f', 6 )
I apologize for the traffic, but at least we now have more accurate logging for
MoNav :) .
--
Beste Grüße,
Best regards,
ce