[分享]编写你自己的gps应用程序

<P>从codeguru转过来的，和大家一起学习下</P>
<P>Writing Your Own GPS Applications: Part I
</P>
<P><b>Environment:</b>  C#, VB.NET, Visual Studio 2002/2003/2005, .NET Framework, .NET Compact Framework </P>

<P><IMG src="http://www.codeguru.com/dbfiles/get_image.php?id=8079;lbl=GPSPOCKETPC3_JPG;ds=20040901"></P>
<P>"I am continually amazed by how little code is required to use atomic clocks in satellites 11,000 miles above my head."</P>
<P>What is it that GPS applications need to be good enough to handle in-car navigation? Also, how does the process of interpreting GPS data actually work? In this two-part series, I will cover both topics and give you the skills you need to write a commercial-grade GPS application that works with a majority of GPS devices in the industry today.</P>
<H3>One Powerful Sentence</H3>
<P>This first part in the series will explore the task of interpreting raw GPS data. Fortunately, the task is simplified thanks to the National Marine Electronics Association (www.nmea.org), which introduced a standard for the industry now in use by a vast majority of GPS devices. To give developers a head start, I chose to use some Visual Studio.NET source code from my "GPS.NET Global Positioning SDK" component. (The code is stripped of features like multithreading and error handling for brevity.)</P>
<P>NMEA data is sent as comma-delimited "sentences" that contain information based on the first word of the sentence. There are over fifty kinds of sentences, yet an interpreter really only needs to handle a few to get the job done. The most common NMEA sentence of all is the "Recommended Minimum" sentence, which begins with "$GPRMC." Here is an example:</P><PRE>$GPRMC,040302.663,A,3939.7,N,10506.6,W,0.27,358.86,200804,,*1A</PRE>
<P>This one sentence contains nearly everything a GPS application needs: latitude, longitude, speed, bearing, satellite-derived time, fix status, and magnetic variation.</P>
<H3>The Core of An Interpreter</H3>
<P>The first step in making an NMEA interpreter is writing a method that does two things: separating each sentence into its individual words and then examining the first word to figure out what information is available to extract. Listing 1-1 shows the start of the interpreter class.</P>
<P><B>Listing 1-1:</B> The core of an NMEA interpreter is a function which divides NMEA sentences into individual words.</P><PRE>'*******************************************************
'**  Listing 1-1.  The core of an NMEA interpreter
'*******************************************************
Public Class NmeaInterpreter
 ' Processes information from the GPS receiver
 Public Function Parse(ByVal sentence As String) As Boolean
  ' Divide the sentence into words
  Dim Words() As String = GetWords(sentence)
  ' Look at the first word to decide where to go next
  Select Case Words(0)
   Case "$GPRMC"    ' A "Recommended Minimum" sentence was found!
    ' Indicate that the sentence was recognized
    Return True
   Case Else
    ' Indicate that the sentence was not recognized
    Return False
  End Select
 End Function
 ' Divides a sentence into individual words
 Public Function GetWords(ByVal sentence As String) As String()
  Return sentence.Split(","c)
 End Function
End Class
</PRE>
<P><B>About the Author</B>
Jon Person is the author of the award-winning "<a href="http://www.gpsdotnet.com/" target="_blank" >GPS.NET Global Positioning SDK</A>" component and has consulted for military companies and the Forest Service on GPS solutions. Jon frequently writes about GPS application development and welcomes all questions and comments at <a href="mailtinfo@gpsdotnet.com" target="_blank" >info@gpsdotnet.com</A>. </P>
<P>第一部分的代码：</P>
<P><B>Downloads</B>

<LI><a href="http://www.codeguru.com/dbfiles/get_file/PartOneListings.zip?id=8079;lbl=PARTONELISTINGS_ZIP;ds=20040901" target="_blank" >PartOneListings.zip</A> - Listings for Part One (Visual Basic.NET)
</LI>

The Core of An Interpreter (Continued)
<P>The next step is to perform actual extraction of information, starting with latitude and longitude. Latitude and longitude are stored in the form "DD°MM'SS.S" where D represents hours (also called "degrees"), M represents minutes, and S represents seconds. Coordinates can be displayed in shorthand, such as "DD°MM.M" or even "DD°." The fourth word in the sentence, "3939.7," shows the current latitude as hours and minutes (39°39.7'), except the numbers are squished together. The first two characters (39) represent hours and the remainder of the word (39.7) represents minutes. Longitude is structured the same way, except that the first three characters represent hours (105006.6'). Words five and seven indicate the "hemisphere," where "N" means "North," "W" means "West," and so forth. The hemisphere is appended to the end of the numeric portion to make a complete measurement.</P>
(continued)
<P>
<P>I've found that NMEA interpreters are much easier to work with; they are event-driven. This is because data arrives in no particular order. An event-driven class gives the interpreter the most flexibility and responsiveness to an application. So, I'll design the interpreter to report information using events. The first event, PositionReceived, will be raised whenever the current latitude and longitude are received. Listing 1-2 expands the interpreter to report the current position.</P>
<P>Listing 1-2: The interpreter can now report the current latitude and longitude.</P>'********************************************************'**  Listing 1-2.  Extracting information from a sentence'********************************************************Public Class NmeaInterpreter ' Raised when the current location has changed Public Event PositionReceived(ByVal latitude As String, _                               ByVal longitude As String) ' Processes information from the GPS receiver Public Function Parse(ByVal sentence As String) As Boolean  ' Look at the first word to decide where to go next  Select Case GetWords(sentence)(0)   Case "$GPRMC"    ' A "Recommended Minimum" sentence was found!    Return ParseGPRMC(sentence)   Case Else    ' Indicate that the sentence was not recognized    Return False  End Select End Function ' Divides a sentence into individual words Public Function GetWords(ByVal sentence As String) As String()  Return sentence.Split(","c) End Function ' Interprets a $GPRMC message Public Function ParseGPRMC(ByVal sentence As String) As Boolean  ' Divide the sentence into words  Dim Words() As String = GetWords(sentence)  ' Do we have enough values to describe our location?  If Words(3) <> "" And Words(4) <> "" And Words(5) <> "" _                    And Words(6) <> "" Then   ' Yes. Extract latitude and longitude   Dim Latitude As String = Words(3).Substring(0, 2) ; "0"      ' Append hours   Latitude = Latitude ; Words(3).Substring(2) ; """"      ' Append minutes   Latitude = Latitude ; Words(4)    ' Append the hemisphere   Dim Longitude As String = Words(5).Substring(0, 3) ; "0"      ' Append hours   Longitude = Longitude ; Words(5).Substring(3) ; """"      ' Append minutes   Longitude = Longitude ; Words(6)     ' Append the hemisphere   ' Notify the calling application of the change   RaiseEvent PositionReceived(Latitude, Longitude)  End If  ' Indicate that the sentence was recognized  Return True End FunctionEnd Class
<P>One thing to watch out for here is that some GPS devices will report blank values when no information is known. Therefore, it's a good idea to test each word for a value before parsing. If you need to type the degree symbol (°), hold down the Alt key and type "0176" on the numeric keypad.</P>

<P>贴在这里不太好看，还是把地址贴出来，大家自己去看吧</P>
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