[Error] expected unqualified-id before 'public'

Question

public class KalmanLatLong {
private final float MinAccuracy = 1;

private float Q_metres_per_second;
private long TimeStamp_milliseconds;
private double lat;
private double lng;
private float variance; // P matrix. Negative means object uninitialised. NB: units irrelevant, as long as same units used throughout

Answer 1

If you mean C++, you are not using the access specifier keywords correctly.
class Class {
private:
int prop1;
int prop2;
public:
int prop3;
private:
int prop4;
};
You can't use an access specifier on the class. The specifiers are used to group the properties and methods of the class.