MongoDB C# offi : Lat, Lng as doubles are stored at precision13 and rounding? - mongodb

Question

I store lat and lng as double in MondoDB and C# official driver. I have problems with my points that are not matching the right places after a rounding of double values internaly in MondoDB(by driver or intern). I have searched all posible rounding before post to the Repository but haven´t found anything. Why not use decimal, because Query.Near use doubles.
v1.6.5 ; C#v0.11 ; Win64.
XX.444057295828145;XX.63416004180907 Captured as string
XX.444057295828145;XX.63416004180907 Receipt by Repository before Save method
Inside MongoDB and returned :
XX.4440572958281, XX.6341600418091, Saved and returned (only 13 after dot)
This is resulting to a moved map. Advices. Thanks.
I use this Update method
public void SaveGeoPoints(String id, String lat, String lng)
{
//--> XX.444057295828145;XX.63416004180907
Db.repository.Update(
Query.EQ("_id", id),
Update.Set("Lat", double.Parse(lat))
.Set("Lng", double.Parse(lng)));
}

Answer 1

the default format for Console.WriteLine prints only 13 digits after the decimal point. The following code snippet shows this
var test = "10.444057295828145";
Console.WriteLine(test);
var testInDouble = double.Parse(test);
Console.WriteLine(testInDouble);
Console.WriteLine(testInDouble.ToString("R"));
Console output is
10.444057295828145
10.4440572958281
10.444057295828145

Answer 2

I am unable to reproduce what you are describing. I can save and retrieve those values from MongoDB without loss of precision. Here's the test code I used:
http://www.pastie.org/1599603
As an additional test I verified the contents of the documents in the database using the Mongo shell.
There must be some rounding going on somewhere else. I would check all places where you convert back and forth between doubles and strings.

Answer 3

Here's another test that uses the Update.Set method:
http://www.pastie.org/1599777
I still see no loss of precision when storing values to the database and reading them back.
You don't say what your typical values of XX are. I'm using 12 in the test.
Note that double only has a total precision of slightly over 15 decimal digits (doesn't matter where the decimal point is), so the larger your values of XX the less precision that is left over to use to the right of the decimal point.
If you are simply exceeding the precision limits of the 64 bit IEEE double type that is not anything to do with either the C# driver or MongoDB.

Answer 4

Thanks Sam and bugai for this help. Of course all answers are corrects. It's a limits of C# double and printings in C# like point Bugai in upper post. I post here the logical used to others users like me that lost the North with this amount of decimals. Not use double? is not accepting round-trip format. Use round-trip formatter in your outputs to Api. Thanks for comprension. Get Location now having 15 decimals. P.R
public class BsonCentroid
{
[BsonId]
public String Id { get; set; }
public String Location
{
get { return String.Format("{0};{1}", Lat.ToString("r"), Lng.ToString("r")); }
}
public double Lat { get; set; }
public double Lng { get; set; }
}