GPS, NMEA, WGS-84 and VB.NET

	GPS, NMEA, WGS-84, GIS and VB.NET By Thomas Tingsted Mathiesen (tma@tma.dk) I made this page because I have almost surfed my self to death trying to piece all the different information together - It's out there, but somewhat scattered all over. My goal is to develop a VB.NET class library where all functionality conserning positioning, Datum conversion, Grid transformation and direct serial interface to an external USB/Serial GPS reciever. My Linked-In profile My company - Gaudio ApS My FaceBook profile
Last changes made: 16 JUL 2008

Index

Coordinate formats

Get the NMEA linies from the GPS reciever

Convert NMEA format to Decimal format

Convert from Decimal format to NMEA/WGS-84

Calculate distance between two coordinates/locations

Calculate direction in degrees between two coordinates

Magnetic deviation/declination

Using maps.google.com to view Longitude/Latitude

Sirf Star III stability test

Links & references

The .NET GPS API ready for download

Go to my GPS API page and check out the features :)

Goto: .NET GPS API

Contribute to the build of a comprehensive list of GPS/NMEA dumps.
See sample data from around the globe using different GPS systems.

Submit serial output, or check out the other dumps

Please give your comment/rating at the bottom of this page

Coordinate formats

NMEA(WGS-84)	5601.0318,N 01211.3503,E
Decimal format	56.01755 12.19302

Get the NMEA linies from the GPS reciever
(Link to all NMEA commands, line descriptions, fields etc. http://www.gpsinformation.org/dale/nmea.htm)
I use the $GPGGA line for getting the current position data.
Example: $GPGGA,165113.000,5601.0318,N,01211.3503,E,1,07,1.2,22.4,M,41.6,M,,0000*60

As you can see my position is:(Denmark)
Longitude: 5601.0318 N
Latitude: 01211.3503 E

Textfile with a NMEA/GPS dump from the terninal (Download)

Convert the recieved NMEA(WGS-84) location to a decimal number.
I had some trouble finding the correct formula for doing this, but I finally found this one, and it works like a charm.
You have to do this conversion, due to the fact that all maps(that I know of) uses decimal longitudes and latitudes.(maps.google.com, Mappoint, Autoroute etc)
If you should make the same mistake as I did, trying to type the location on maps.google.com without this conversion, you will get a position near to you, but not close enough.

Formula (5601.0318) = 56+(1.0318/60)
Example:

    Private Function toDecimal(ByVal Pos As String) As Double
        'Pos="5601.0318"
        Dim PosDb As Double = CType(Replace(Pos, ".", ","), Double) 'Replace . with , (Used in danish doubles)
        Dim Deg As Double = Math.Floor(PosDb / 100)
        Dim DecPos As Double = Math.Round(Deg + ((PosDb - (Deg * 100)) / 60), 5)
        Return DecPos '=56.0172
    End Function

Remember:
If you are located on the Western hemisphere (America/US) or southern hemisphere (Australia), convert to negative position.(Decpos = Decpos * -1)

Thank's to Leonardo for the currect formula.

Convert decimal location to NMEA/WGS-84 format.

Public Enum enumLongLat
    Latitude = 1
    Longitude = 2
End Enum
Public Enum enumReturnformat
    WithSigns = 0
    NMEA = 1
End Enum

Public Function DecimalPosToDegrees(ByVal Decimalpos As Double, ByVal Type As enumLongLat, ByVal Outputformat As enumReturnformat, Optional ByVal SecondResolution As Integer = 2) As String
        Dim Deg As Integer = 0
        Dim Min As Double = 0
        Dim Sec As Double = 0
        Dim Dir As String = ""
        Dim tmpPos As Double = Decimalpos
        If tmpPos < 0 Then tmpPos = Decimalpos * -1 'Always do math on positive values

        Deg = CType(Math.Floor(tmpPos), Integer)
        Min = (tmpPos - Deg) * 60
        Sec = (Min - Math.Floor(Min)) * 60
        Min = Math.Floor(Min)
        Sec = Math.Round(Sec, SecondResolution)

        Select Case Type
            Case enumLongLat.Latitude '=1
                If Decimalpos < 0 Then
                    Dir = "S"
                Else
                    Dir = "N"
                End If
            Case enumLongLat.Longitude '=2
                If Decimalpos < 0 Then
                    Dir = "W"
                Else
                    Dir = "E"
                End If
        End Select
        Select Case Outputformat
            Case enumReturnformat.NMEA
                Return AddZeros(Deg, 3) & AddZeros(Min, 2) & AddZeros(Sec, 2)
            Case enumReturnformat.WithSigns
                Return Deg & "°" & Min & """" & Sec & "'" & Dir
            Case Else
                Return ""
        End Select
    End Function
    
    
    Public Function AddZeros(ByVal Value As Double, ByVal Zeros As Integer) As String
        If Math.Floor(Value).ToString.Length < Zeros Then
            Return Value.ToString.PadLeft(Zeros, CType("0", Char))
        Else
            Return Value.ToString
        End If
    End Function

You can get the return value in two different formats.
NMEA = 01211.3487,N
WithSigns = 56°1"3.18'N

Example:

Dim conV As New NMEA_Transform
MsgBox(conV.DecimalPosToDegrees(12.19302, enumLongLat.Latitude, enumReturnformat.NMEA))

Returns: 01211.3487,N

Calculate the distance between two coordinates
I found an article on codeproject.com by Gary Dryden, written in C#.
Remember to convert your position to Decimals first.
I converted it to VB.NET like so:

Imports System
Imports System.Text

Public Class Distance
    Public Function Calc(ByVal Lat1 As Double, ByVal Long1 As Double, ByVal Lat2 As Double, ByVal Long2 As Double) As Double
        Dim dDistance As Double = Double.MinValue
        Dim dLat1InRad As Double = Lat1 * (Math.PI / 180)
        Dim dLong1InRad As Double = Long1 * (Math.PI / 180)
        Dim dLat2InRad As Double = Lat2 * (Math.PI / 180)
        Dim dLong2InRad As Double = Long2 * (Math.PI / 180)
        Dim dLongitude As Double = dLong2InRad - dLong1InRad
        Dim dLatitude As Double = dLat2InRad - dLat1InRad
        Dim a As Double = Math.Pow(Math.Sin(dLatitude / 2), 2) + Math.Cos(dLat1InRad) * Math.Cos(dLat2InRad) * Math.Pow(Math.Sin(dLongitude / 2), 2)
        Dim c As Double = 2 * Math.Atan2(Math.Sqrt(a), Math.Sqrt(1 - a))
        Const kEarthRadiusKms As Double = 6376.5
        dDistance = kEarthRadiusKms * c
        Return dDistance
    End Function
End Class

Returns distance in Kilometers.
Example: MyDistance = Calc(56.0176,12.19302,56.0176,12.19301)

Calculate direction in degrees between two coordinates
This code was by far the hardest to dig up, but thanks to Jim Mischel and his fantastic article on informit.com it came to work.

Public Class Form1
    Structure wptType
        Dim lat As Double
        Dim lon As Double
        Dim ele As Double
    End Structure
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim Distance As Double = 0
        Dim Course As Double = 0
        Dim Source As wptType
        Dim Destination As wptType
        Source.lat = 56
        Source.lon = 12
        Destination.lat = 56
        Destination.lon = 11
        GetCourseAndDistance(Source, Destination, Course, Distance)
        MsgBox(Course)
    End Sub
    Sub GetCourseAndDistance(ByVal pt1 As wptType, ByVal pt2 As wptType, ByRef course As Double, ByRef dist As Double)
        ' convert latitude and longitude to radians
        Dim lat1 As Double = DegreesToRadians(CDbl(pt1.lat))
        Dim lon1 As Double = DegreesToRadians(CDbl(pt1.lon))
        Dim lat2 As Double = DegreesToRadians(CDbl(pt2.lat))
        Dim lon2 As Double = DegreesToRadians(CDbl(pt2.lon))

        ' compute latitude and longitude differences
        Dim dlat As Double = lat2 - lat1
        Dim dlon As Double = lon2 - lon1

        Dim distanceNorth As Double = dlat
        Dim distanceEast As Double = dlon * Math.Cos(lat1)

        ' compute the distance in radians
        dist = Math.Sqrt(distanceNorth * distanceNorth + distanceEast * distanceEast)

        ' and convert the radians to meters
        dist = RadiansToMeters(dist)

        ' add the elevation difference to the calculation
        Dim dele As Double = CDbl(pt2.ele - pt1.ele)
        dist = Math.Sqrt(dist * dist + dele * dele)

        ' compute the course
        course = Math.Atan2(distanceEast, distanceNorth) Mod (2 * Math.PI)
        course = RadiansToDegrees(course)
        If course < 0 Then
            course += 360
        End If
    End Sub
    Function DegreesToRadians(ByVal degrees As Double) As Double
        Return degrees * Math.PI / 180.0
    End Function

    Function RadiansToDegrees(ByVal radians As Double) As Double
        Return radians * 180.0 / Math.PI
    End Function

    Function RadiansToNauticalMiles(ByVal radians As Double) As Double
        ' There are 60 nautical miles for each degree
        Return radians * 60 * 180 / Math.PI
    End Function

    Function RadiansToMeters(ByVal radians As Double) As Double
        ' there are 1852 meters in a nautical mile
        Return 1852 * RadiansToNauticalMiles(radians)
    End Function
End Class

Copy code into a form containing a Button1.
Edit values in Button1_Click:
Source.lat
Source.lon
Destination.lat
Destination.lon

Maps and magnetic deviation/declination
If you need to calculate the magnetic deviation, use this link into NOAA.
NOAA's website (Declination calculator)
Under normal circumstances you will not need to manually do this calculation - your GPS reciever should do this for you.

Using Google maps to position your self with Longitude/Latitude
Pretty simple really, just:
http://maps.google.com?q=56.01717,12.18909

How stable is my GPS reciever with the new SirfStar III chipset
I must admit that this reciever performs beyond what I had expected.
This is a simple graph displaying how the position "jumps" around, when stationary.
(X axis=2,78 Meters pr. square, Y axis=1,56 Meters pr.square, with 62 recieved GPS fixes)

GPS map, stationary, using SirfStar III chipset

GPS map, stationary, using SirfStar III chipset

This one is using the old SirfStar II chipset.
GPS map, stationary, using SirfStar II chipset

Links to usefull pages and references

Name	Link
National Geophysical Data Center(NGDC)	http://www.ngdc.noaa.gov/seg/geomag/jsp/struts/calcDeclination
GLOSSARY OF GPS TERMINOLOGY	http://www.rbf.com/cgcc/glossary.htm
Wikipedia's story on Magnetic declination	http://en.wikipedia.org/wiki/Magnetic_variation
NMEA Sentence information	http://www.tma.dk/gps/nmeastrings.htm
Deviation (Graphs)	http://swepos.lmv.lm.se/english/sa.htm
Google groups on the subject	http://groups.google.dk/group/Google-Maps-API/browse_thread/thread/7b36c0f8b6a0d592/ab5580ea1b6adaaf?lnk=st&q=convert+NMEA+to+WGS-84+decimal&rnum=2&hl=da#ab5580ea1b6adaaf
Jim Mischel's fantastic article on 'Computing Track Information in GPS'	http://www.informit.com/guides/content.asp?g=dotnet&seqNum=513&rl=1

Source:maps.google.com, my location using my GPS reciever

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Date	Name	Rate	Message	Homepage
06-03-2010	Shameem	9	Hi Mr Mathiesen,Im writing an app in vb6.how do i get to show my small vehicle icon at my gps location,just like u have the red dot on yours in google earth.Im referencing the googleearth.exe in my project. Thanx for any assistance.
24-02-2010		9
23-02-2010		9
20-02-2010		7
17-11-2009		9
28-10-2009	Artur	9	very useful	www.matematyka-blog.info
04-08-2009	MMasri	9
19-07-2009	Johnn	9
15-07-2009		9
02-05-2009	Kevin	9	Excellent Application Thomas.
31-03-2009	John	9
09-03-2009	zadania domowe	8	thanks	www.trudne.pl
01-03-2009	Chris	8	Looks good but I cannot register the DLL in Windows Vista. The msg from regsvr32 is The module tmagpsapi.dll was loaded but the entry-point DllRegisterServer was not found. Any ideas please?
17-02-2009	James	9	This page has been great in helping build an autoilot for my rc plane.
12-02-2009		7
12-02-2009	Rickard	9	Sweet code! =) I had helped me with quite some stuff over the past month. Thanx alot.	www.virtualpunk.com
09-01-2009	Medhat Dessouki	9	Very good work, efforts & Info. Well done.	m_dessouki2002@yahoo.com
31-12-2008	Rasmus	8	Tusen tack Thomas. All information på ett och samma ställe.
02-12-2008	Chris	9	Great work here. Do you by chance have or know where to get the formula to convert from WGS84 to SPCS83 ?	www.caroneast.com
01-12-2008	Raman	9	Thomas, thank you so much for your efforts and hardwork to put together all. I am learning and novice programmer. I wanted to implement RealTime Distance caluculation, when I connect to my LapTop thru COM port. how can I do that? Can you give me some thoughts and suggestions please.
01-12-2008	vpolv	9	thanks. The info you provide is excellent and easy to understand.. thanks again...
01-12-2008	vpolv	9	thanks. The info you provide is excellent and easy to understand.. thanks again...
25-10-2008		9
07-10-2008	David Leach	8	In the NMEA to Decimal function, you need to check and convert to positive number before the calculations (FLOOR) are started. Other wise the floor of -9.5 returns -10 and the rest of the calculation gets off by 1. I also took off the first part that strips and replaced the . with a ,. It didnt seem to work as you intended and caused the whole string to look like a giant integer and wasnt necessary with my data anyway. Private Function toDecimal(ByVal Pos As String) As Double Pos=5601.0318 Dim PosDb As Double = CType(Replace(Pos, ., ,), Double) Replace . with , (Used in danish doubles) Dim negbool As Boolean If (Pos 0) Then negbool = True Dim PosDB As Double = Math.Abs(CType(Pos, Double)) Dim Deg As Double = Math.Floor(PosDB / 100) Dim DecPos As Double = Math.Round(Deg + ((PosDB - (Deg * 100)) / 60), 5) If negbool Then Return -DecPos Else Return DecPos =56.0172 End If End Function
25-09-2008		7
15-07-2008	Luca	9	Hi Thomas, very useful page, thankyou! But I think there may be something wrong in DecimalPosToDegrees (or something I didnt understand): the NMEA result WITHSIGNS you show (56°13.18N) seems not to be correct since the WGS84 format value is 5601.0318 (where .0318 are decimal of minutes, not seconds). Moreover, in the NMEA output format case statement, the function should convert seconds to decimal of minutes. Am I wrong? Thank you again. Luca Hi Luca, I will check up on it.. Thanks... /Thomas
02-07-2008	winbell@hotmail.com	9	good job
20-06-2008	Jesus	9	Hi, Thomas. Thank you very much for your article, has helped mae a lot. Just one (maybe obvious) note. To convert Longitude you have to take the three first digits, as degress, while in Latitude you only take the first two ones. It has take a little bit to me to realize why I do not get the rigth Longitude. Thank you again. Jesus.
14-06-2008	Nicola	9	wow
12-06-2008	Gaurav Agrawal	7	Article is excellent.could anyone give me source code for conversion from NMEA format(WGS84) to Meter Format(British National grid map) or Decimal to Meter conversion.
11-06-2008		9	just what I was looking for!
19-05-2008	Alex	9	very very good
15-05-2008	Yael	9	Thank you very much
08-05-2008	nikos	9	Thank you Very Match
22-03-2008	dzwonki	7	thx	www.dzwonker.pl
05-03-2008	andrew	7	Brilliant, thanx a lot ! You dont by any chance also have a function for calulating a new posision given a position and a distance ? /andrew Hi Andrew No, sorry I haven't got that formula, but if you or anybody else in here knows of one I would very much like to post it here. /Thomas Mathiesen
03-03-2008	N. Tabatabai	7	Hi Thomas, first of all, Im happy to see developers helping each other. Information I get, was so helpfull and Exactly what I need. thank you
21-02-2008	Joachim	9	Hi Thomas, very usefull information, thank you. Dont know if Im wrong, but: the GPS spec is nutty ... since there are only 90 degrees of latitude and 180 degrees of longitude the latitude values in degrees never have 3 digits ... so the spec for latitude is: ddmm.mmmm and longitude is: dddmm.mmmm In DecimalPosToDegrees you always add 3 zeros, this can cause problems
11-02-2008	Bek	9	Thank you very much, Thomas Mathiesen !!!!
02-02-2008		8
12-01-2008	Azad	9	Hi Thomas, This is absolutely a great stuff. You said for Western and Southern hemispheres multiply with -1. So I have to change the source code if I change hemispheres? And is there any separate source code file for the dll? Hi Azad, No you don't have to change the code when changing hemisphere. With regars to the sourcecode of the tmagpsapi.dll, I do not wish to share this (Write me on tmatma.dk if you whant an explanation why) Kind regards. Thomas Mathiesen
03-01-2008	max	9	:) i like very helpful (i am writing a program which processes Navman gps data logs :D)
13-12-2007	N. V. Jensen	9	Det var lige hvad jeg manglede.	nvjensen.dk
05-12-2007	hnguyen	7	great Job. Anyway, the decimal format is not correct, should be xx.xxxxx for Lat and yyy.yyyyy for Lon Please change and notice your update Appreciate it Thanks Thomas
26-11-2007	Rob	9	Great, was looking for this info a long time
30-10-2007	Dan	9	Thanks for the help. Really great stuff with the Calculate distance and direction in degrees between two coordinates
01-10-2007		9
18-09-2007		8
18-09-2007	Isaac	9	FANTASTIC!! Thanks so much! Really Great stuff!
11-09-2007	B.	9	You are the MAN..!! Good to see developers helping each other :)
05-09-2007	Peter	9	Great collection of useful information and a nice API as well!
21-08-2007	Cranesfreak	8	The info you provide is excellent and easy to understand
16-08-2007	Fred	1	You wrote this huge error: Longitude is the Y axis (north-South) - False Latitude is the X axis (West-East) - False Please change also your worldmap picture... The real definition is the opposite! Latitude gives the location of a place on Earth north or south of the equator. Longitude is the east-west geographic coordinate. Jesus - you are right Fred - Must have been sleeping that day, sorry about that - I have removed it. (Update will follow) Thanks Thomas Mathiesen
14-08-2007	Andreas	9	Excellent Page with most useful information collected on one website! Thx a lot.
08-08-2007		9
02-08-2007	Rosli	9	I try to fine conversion between wgs84 to local coordinates software for all countryplease help me for my problem	liroii@yahoo.com
26-07-2007	Derrick	9	Good work Thomas and thank you. I was going nuts trying to understand why Google Earth decimal would not work with my NMEA output.
15-07-2007		7
01-07-2007	Harikumar	8	Very good collection of most important codes for customised GPS /embedded GPS software developers
23-06-2007	Can	9	After searching some minutes I ended up with this wonderful page. All I need to know was here. Thank you Thomas.
21-06-2007	Paul H-T	8	I translated this info Excel functions, but my result was around 2000 km, when it should be under a km! Do you have an Excel version of these formulas available? I have some sample data and the formulas in a file that I could send to someone to double check.	www.iwireless.com
14-05-2007	rexSalius	9	Wonderful site, Thomas. Why didnt I find it earlier.... -) Thanks
12-05-2007	David	9	Thank you very much for this info. It is very usefull and it avoided me to expend a lot of time in analyze. Thank you again Thomas.
09-05-2007	Randy	9	Wonderful... !!! Is the GPS Infromation app available. I would like to look at the drift of my GPS unit.
06-05-2007	Javier	6	Thomas, I cant connect my usb garmin gps, any suggestions ? Hi Javier I have not testet the software on Garmin (Don't have one) - so if any of you could send me some raw Garmin GPS data I will try to support it. - Thomas Mathiesen	www.netgis.com.ar
04-05-2007	Rick C.	9	Do you have any examples for geofencing calculation? Hi Rick No, I have not, but the subject is exciting, and I will look into it. Are you looking for ways to calculate when the position is outside a circle or shape? Regards -Thomas Mathiesen
01-05-2007	Robertr Brenner	8	Amazing what love for a hobby can create!
15-04-2007		9
14-04-2007	Malcolm Haines	7	Its great to see all this info together. i too almost surfed myself stupid trying to piece it all together. Thanks Thomas!
19-03-2007	Aleksander Posmyk	9	lot of useful information. thx	posmyk.info
19-03-2007		9
10-03-2007	Maksim Sviatlitski	9	Thomas, thank you so much for your efforts! This page was exactly what I was looking for!	www.agile-sys.com
28-02-2007	Adam OConnor	9	Absolutely Exceptional
02-01-2007		9
01-01-2007		9
24-12-2006		9
22-12-2006	Nasrullah	8	Lot of Thanks	www.4bgroup.com
15-12-2006	Ubi Sumus	6	DecimalPosToDegrees has a tiny bug. In Germany we use , for decimal separator not a . NMEA gets confused by an additional , We use ,(comma) for decimal seperation as you do, that is why I replace the .(dot) with a ,(comma) - Thomas
11-12-2006		8
09-12-2006	LazBoy	8	Thanks a billion!! Very informative page.
27-11-2006	landofshadows	7	What I was after was how to make an Excel forumla to convert Latitude & Longitude into a English Postcode... You see we have the Cordinates but need to see the location by postcode... and at the moment I am having to check them one at a time, 1,500 per day... any one able to help me email me please landofshadowslycos.co.uk (Subject GPS)
27-11-2006		7
24-11-2006		9
15-11-2006		9
14-11-2006		9	Thanks for your nice work.
27-10-2006		9
16-10-2006	Leonardo	5	Nice page, but, I think that in GPGGAs lat/lon the minutes are in radians, but the value after the comma means the decimal part of the minutes, not the seconds. see http://aprs.gids.nl/nmea/#gga Try the formula:DEGREES+(MINUTES+/60) - 56+(01.0318/60) in your example you are likely to get beter results leonardo(at)novageo(dot)pt -Thank you Leonardo, I have corrected the formula/Thomas Mathiesen
15-10-2006		7	Unfortunatly the link http://www.gpsinformation.org/dale/nmea.htm isnt working - Is up again/Thomas Mathiesen
11-10-2006		6	good work..
09-10-2006		5
06-10-2006		9	Ive been looking for this information for some time now, thank a lot guys the content is great.
04-10-2006		9
01-10-2006		9	Very nice... Had some useful info :)
06-09-2006		9
29-08-2006	Peter Cranstone	9	Great job. Exactly what we needed. Peter.Cranstone5o9inc.com
14-08-2006		8	Thanks. This realy helped me a lot.
09-08-2006		9

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