www.pudn.com > SerialGPS.zip > ReadTracks.m, change:2002-12-12,size:4101b

function tracks = ReadTracks(gps) 
% ReadTracks Read all the track data from the GPS 
 
global pid_command_data cmd_transfer_trk pid_xfer_cmplt pid_trk_hdr 
global pid_ack_byte pid_nak_byte pid_trk_data 
 
tracks = []; 
 
% Tell the GPS to send the track data to us 
WritePacket(gps, pid_command_data, cmd_transfer_trk, 'int16'); 
 
% Read standard beginning packet -- the count is the number of packets to 
% follow. This stream of packets may consist of multiple tracks. 
[id, packets] = ReadPacket(gps, 'int16'); 
 
% The latitude and longitude are stored on the GPS as "semicircles", where 
% there are 2^31 semicircles in 180 degrees. We want to return the data as 
% degrees, so compute a conversion factor. 
semi2deg = (180 / (2^31)); 
 
% Until the transfer is complete 
n = 0; 
count = 0; 
while (pid_xfer_cmplt ~= id) 
    count = count + 1; 
    [id, sz] = ReadPacketHeader(gps); 
    csum = -1; 
    disp(['Reading record ' num2str(count) ' of ' num2str(packets+1) '.']); 
     
    if (pid_trk_hdr == id) 
        % Resize the previous track, if necessary 
        if (n > 0) 
            tracks(n).waypoints = tracks(n).waypoints(1:w); 
        end 
         
        % Start a new track 
        n = n + 1; 
        w = 0; 
         
        % Read the header data, which consists of a numeric identifier and 
        % a null-terminated comment  
        tracks(n).display = fread(gps, 1);  
        tracks(n).color = fread(gps, 1); 
        term = get(gps, 'Terminator'); 
        set(gps, 'Terminator', 0); 
        tracks(n).comment = fgets(gps); 
        set(gps, 'Terminator', term); 
         
        % Allocate space for the track packets 
         tracks(n).waypoints(packets) = struct('lat', [], 'long', [], 'time', [], ... 
                                               'alt', [], 'depth', [], 'newtrk', []); 
         
        % Compute the checksum 
        csum = ComputeChecksum(id, [tracks(n).display tracks(n).color tracks(n).comment 0]); 
         
        disp(['Starting new track. Name: "' tracks(n).comment '"']); 
         
    elseif (pid_trk_data == id) 
        % Read track log data into the current track 
        w = w+1; 
         
        % Read the waypoint data 
        [ tracks(n).waypoints(w).lat, d ] = ReadData(gps, id, 1, 'int32'); 
        data = d; 
        tracks(n).waypoints(w).lat = tracks(n).waypoints(w).lat * semi2deg; 
         
        [ tracks(n).waypoints(w).long, d ] = ReadData(gps, id, 1, 'int32'); 
        data = [ data d ]; 
        tracks(n).waypoints(w).long = tracks(n).waypoints(w).long * semi2deg; 
         
        [ tracks(n).waypoints(w).time, d ] = ReadData(gps, id, 1, 'uint32'); 
        data = [ data d ]; 
         
        [ tracks(n).waypoints(w).alt, d ] = ReadData(gps, id, 1, 'float32'); 
        data = [ data d ]; 
        % Convert altitude from meters to feet 
        tracks(n).waypoints(w).alt = tracks(n).waypoints(w).alt * 3.28; 
         
        [ tracks(n).waypoints(w).depth, d ] = ReadData(gps, id, 1, 'float32'); 
        data = [ data d ]; 
         
        [ tracks(n).waypoints(w).newtrk, d ] = ReadData(gps, id, 1, 'uint8'); 
        data = [ data d ]; 
        
        % Skip three mysterious bytes 
        d = ReadData(gps, id, 3, 'uint8'); 
        data = [ data d ]; 
 
        % Compute the checksum 
        csum = ComputeChecksum(id, data); 
         
    elseif (pid_xfer_cmplt == id) 
        % Resize the current track 
        tracks(n).waypoints = tracks(n).waypoints(1:w); 
         
        % Read the data in the pid_xfer_cmplt packet 
        data = ReadData(gps, id, sz, 'uint8'); 
        csum = ComputeChecksum(id, data); 
    else 
        WritePacket(gps, pid_nak_byte, id); 
        error(['Unexpected packet id ' num2str(id) ' while reading track log.']);      
    end
     
    cksum = ReadPacketTerminator(gps); 
    if (csum ~= cksum) 
        WritePacket(gps, pid_nak_byte, id); 
        error(['Checksum mismatch in ReadTracks. Expected: ' num2str(csum) ', Received: ' num2str(cksum)]); 
    else 
        WritePacket(gps, pid_ack_byte, id); 
    end 
end