JSBSim Increase Output Directive Precision

[sr71684]

Describe the issue

How do I increase the precision of JSBSim output directive properties?

What is the current behavior?
Precision of position/lat-geod-deg and position/long-gc-deg is very low, at around 4-5 significant figures.

What is the expected behavior?

I want to increase precision to at least 16 significant figures.

[seanmcleod]

Can you post a copy of how you're specifying your outputs?

If you're outputting to a text file then it looks like by default the precision is set to 10.

jsbsim/src/input_output/FGOutputTextFile.cpp

Line 260 in dd97e99

outstream.precision(10);

However for certain categories of properties the precision is increased, e.g.

jsbsim/src/input_output/FGOutputTextFile.cpp

Lines 341 to 364 in dd97e99

	if (SubSystems & ssPropagate) {
	outstream.precision(14);
	outstream << delimeter;
	outstream << Propagate->GetAltitudeASL() << delimeter;
	outstream << Propagate->GetDistanceAGL() << delimeter;
	outstream << (radtodeg*Propagate->GetEuler()).Dump(delimeter) << delimeter;
	outstream << Propagate->GetQuaternion().Dump(delimeter) << delimeter;
	FGQuaternion Qec = Propagate->GetQuaternionECEF();
	outstream << Qec.Dump(delimeter) << delimeter;
	outstream << Propagate->GetQuaternionECI().Dump(delimeter) << delimeter;
	outstream << Auxiliary->Getalpha(inDegrees) << delimeter;
	outstream << Auxiliary->Getbeta(inDegrees) << delimeter;
	outstream << Propagate->GetLatitudeDeg() << delimeter;
	outstream << Propagate->GetGeodLatitudeDeg() << delimeter;
	outstream << Propagate->GetLongitudeDeg() << delimeter;
	outstream.precision(18);
	outstream << ((FGColumnVector3)Propagate->GetInertialPosition()).Dump(delimeter) << delimeter;
	outstream << ((FGColumnVector3)Propagate->GetLocation()).Dump(delimeter) << delimeter;
	outstream.precision(14);
	outstream << Propagate->GetEarthPositionAngleDeg() << delimeter;
	outstream << Propagate->GetDistanceAGL() << delimeter;
	outstream << Propagate->GetTerrainElevation();
	outstream.precision(10);
	}

If you specify individual properties as opposed to a category like ssPropogate then it looks like the precision is set to 18.

jsbsim/src/input_output/FGOutputTextFile.cpp

Lines 382 to 385 in dd97e99

	outstream.precision(18);
	for (unsigned int i=0;i<OutputParameters.size();++i) {
	outstream << delimeter << OutputParameters[i]->GetValue();
	}

[bcoconni]

This topic has been moved to JSBSim Discussions as per the issue template statement: Issues that are neither bug reports or feature requests will be transferred to [Discussions](https://github.com/JSBSim-Team/jsbsim/discussions) by the team and the issue will be locked and closed..

Support for this topic will therefore continue within this discussion. Thanks for your understanding.

[CarloBlanch1967]

Hello, I'm experiencing the same issue, but on socket via udp.
Precision of position/lat-geod-deg and position/long-gc-deg is very low, only 5 decimal digits.

How can I increase them?

Thanks,

[seanmcleod]

@CarloBlanch1967 have you looked at the source code to confirm whether the precision is hard-coded for the UDP output option that you're using?

I took a look at the source for the text output option in the comment above.

[CarloBlanch1967]

Yes, but I didn't see anything similar in FGOutputSocket.cpp

Thanks a lot.

[seanmcleod]

So for doubles the field width is hard-coded to 12 and the precision is hard-coded to 7.

jsbsim/src/input_output/FGOutputSocket.cpp

Lines 359 to 360 in 60041d7

	socket->Append(Propagate->GetLocation().GetLatitudeDeg());
	socket->Append(Propagate->GetLocation().GetLongitudeDeg());

jsbsim/src/input_output/FGfdmSocket.cpp

Lines 338 to 350 in 60041d7

	void FGfdmSocket::Append(double item)
	{
	if (buffer.tellp() > 0) buffer << ',';
	buffer << std::setw(12) << std::setprecision(7) << item;
	}
	//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
	void FGfdmSocket::Append(long item)
	{
	if (buffer.tellp() > 0) buffer << ',';
	buffer << std::setw(12) << item;
	}

[CarloBlanch1967]

Thanks a lot!

Worderful work, and quick assistance!

Regards,

[seanmcleod]

Just to be clear, the only work I've done so far is to double check the existing code for the socket option to confirm if the precision is hard-coded and if so what it's currently set to.

One option to allow users to set the precision used by the socket output is to add support for a new element, e.g. precision along these lines.

<output name="localhost" type="SOCKET" port="5138" protocol="UDP" rate="10" precision="8">
   
  <property> velocities/vc-kts </property>
  <property> position/h-sl-ft </property>
  <property> attitude/theta-deg </property>

</output>

[CarloBlanch1967]

OMG, I am completely stupid or this parameter is not documented?
Maybe the first, but I'm very happy , this solved my issue.

Thanks again.

[seanmcleod]

No, you've misunderstood what I was saying. I wasn't saying there is such an option currently.

I was saying that one option we could implement/add "..is to add support for a new element..." is to write some code that would support this new element.

So to be clear, nothing has been solved.

[CarloBlanch1967]

Ok, sorry.
Yes, It would be a good solution.
Thanks again!

[seanmcleod]

@CarloBlanch1967 I've just submitted a pull request - #579 to allow a user to optionally specify the output precision on output sockets.

[dpculp]

I'd like to thank Sean and Bertrand for this. I've been having a loss-of-precision problem with QtJSBSim and couldn't find the source. I had been assuming the loss occurred in the JavaScript, but I was wrong!

With 7 significant digits I was getting latitude and longitude jitters of about 3 to 30 feet respectively. It wasn't noticeable until I used an external viewer like Cesium or FlightGear and could see the jitter.

[bcoconni]

@dpculp, all the credits should go to @seanmcleod as I did not do anything valuable.