Where does this precision loss happen and how to prevent it?

Question

I'm writing a simple tool in Qt which reads data from two GPX (XML) files and combines them in a certain way. I tested my tool with track logs that contain waypoints having 6 decimal digits precision. When I read them from the GPX file, the precision gets reduced to 4 decimal digits (rounded properly). So for example this original tag:

<trkpt lat="61.510656" lon="23.777735">

turns into this when my tool writes it again:

<trkpt lat="61.5107" lon="23.7777">

Debug output shows the precision loss happens on this line:

double lat = in.attributes().value("", "lat").toString().toDouble();

but I can't see why. in is a QXmlStreamReader reading from a text file handle.

Answer 1

It is probably when you are writing the value back to the XML. Please post that code in your question.

If I had a guess before seeing the code, you are using QString::number to convert from the double back to a string. The default precision in the conversion is 6, which corresponds to what you are seeing. You can increase the precision to get all the decimals.