GPS with added Start record and distance travelled

And another update to my playing around with the GPS module. This code adds a more complex display with

Start Position - recorded when you press a button

Current Position, or "No Fix" if no valid satellite data received

Time and Date

Distance travelled from Start to Current in nm



Here's the code:

// GPS with report of No Fix 
// and record/display of start position 
// and distance travelled

#include "SoftwareSerial.h"
#include "U8glib.h"

// GPS 11 RX, 10 TX
SoftwareSerial gps(10, 11);

// LCD 6 SCK(E), 5 MOSI(RW), 4 SS(RS)
U8GLIB_ST7920_128X64_1X lcd(6, 5, 4);

char gpsbuf[200]; // GPS output line
char temp[20] = "No Start posn";
char startlatdeg[20] = "--", startlatmin[20] = "", startlondeg[20] = "--", startlonmin[20] = "";
char startns[2] = "", startew[2] = "";
char curlatdeg[20] = "--", curlatmin[20] = "--", curlondeg[20] = "--", curlonmin[20] = "--";
char curns[2] = "-", curew[2] = "-";
char time[20] = "__", date[20] = "--";
char fix[5];
int startflag = false;

  
void setup()
{
    gps.begin(9600); // start GPS serial
    Serial.begin(9600);
    pinMode(2, INPUT_PULLUP); // LOW = read start position
}

void loop()
{
  float lat;
  float lon;
  float dist;
  
  do // get GPS output, look for $GPRMC
  {  
    getline(gpsbuf);
  } while(strncmp(gpsbuf, "$GPRMC", 6) != 0);
  Serial.println(gpsbuf);
  
  // button press records start position
  if(digitalRead(2) == LOW)
  {
    delay(5); // button bounce
    extfmt(3, "**", gpsbuf, startlatdeg); // read deg lat
    extfmt(3, "  *****", gpsbuf, startlatmin); // read min lat
    extfmt(4, "*", gpsbuf, startns); // NS
    extfmt(5, "***", gpsbuf, startlondeg); // read deg lon
    extfmt(5, "   *****", gpsbuf, startlonmin); // read min lon
    extfmt(6, "*", gpsbuf, startew); // EW
    startflag = true;   
  }
  
  
 // extract current position to cur*** arrays
  extfmt(2, "*", gpsbuf, fix);
  Serial.println(fix);
  
  if(fix[0] == 'A')
  {
  extfmt(3, "**", gpsbuf, curlatdeg); // read deg lat
  extfmt(3, "  *****", gpsbuf, curlatmin); // read min lat
  extfmt(4, "*", gpsbuf, curns); // NS
  extfmt(5, "***", gpsbuf, curlondeg); // read deg lon
  extfmt(5, "   *****", gpsbuf, curlonmin); // read min lon
  extfmt(6, "*", gpsbuf, curew); // EW
  extfmt(1, "**:**", gpsbuf, time);
  extfmt(9, "**-**-**", gpsbuf, date);
  }
  else strcpy(curlatdeg, "No Fix");
  
  
  if(startflag) // if there is a cstart position
  {
    
    lat = atof(startlatdeg) * 60 + atof(startlatmin);
    lat -= atof(curlatdeg) * 60 + atof(curlatmin); // lat in minutes
    lon = atof(startlondeg) * 60 + atof(startlonmin);
    lon -= atof(curlondeg) * 60 + atof(curlonmin); // lon in minutes
    dist = sqrt(lat * lat + lon * lon);
    sprintf(temp, "%d", dist); // print int into temp buffer
  }
   
   lcd.firstPage(); // prepare display
   
   do
   {
     lcd.setFont(u8g_font_helvR08);
     
     // 1st line
     lcd.drawStr(2, 8, "Start");
     
     lcd.drawStr(2, 17, startlatdeg);
     lcd.drawStr(20, 17, startlatmin);
     lcd.drawStr(48, 17, startns);
     
     lcd.drawStr(65, 17, startlondeg);
     lcd.drawStr(87, 17, startlonmin);
     lcd.drawStr(118, 17, startew);
     
     lcd.drawStr(2, 26, "Current");
     
     lcd.drawStr(2, 35, curlatdeg);
     lcd.drawStr(20, 35, curlatmin);
     lcd.drawStr(48, 35, curns);
     
     lcd.drawStr(65, 35, curlondeg);
     lcd.drawStr(88, 35, curlonmin);
     lcd.drawStr(118, 35, curew);
     
     lcd.drawStr(2, 44, "Time");
     lcd.drawStr(65, 44, "Date");
     lcd.drawStr(2, 53, time);
     lcd.drawStr(65, 53, date);
     
     lcd.drawStr(2, 62, "Dist nm");
     lcd.drawStr(44, 62, temp);
     
   } while(lcd.nextPage());   
}

// get a line from the GPS
void getline(char *buf)
{
  int bp = 0;
  char c;
  
  do
  {
    c = gps.read(); // -1 if no char available
    if(c == -1) continue;
    buf[bp++] = c;
  } while(c != '\n');
  buf[bp] = '\0';
}

// find field, extract in fmt from inbuf to outbuf
// fmt * = copy inbuf to outbuf, else copy fmt to outbuf
void extfmt(int field, char *fmt, char *inbuf, char *outbuf)
{
  int f = 0; // field count, common & input pointers
  int fp = 0;
  int op = 0;
  int ip = 0;
   
  // find field and ip start
  while(f != field)
  {
    while(inbuf[ip++] != ',');
    f++;
  }
  
 // format from ip
    while(fmt[fp] !='\0')
    {      
      if(fmt[fp] == ' ') ip++; // skip along
      else if(fmt[fp] == '*')
      {
        outbuf[op++] = inbuf[ip++]; // copy in to out
      }
      else
      {
        outbuf[op++] = fmt[fp]; // insert format
      }
      fp++;
    }
  outbuf[op] = '\0'; // add end of string
}