Below is a comparison between the barometric height above ground (from start), which happen to be also around 0 meters above sea level, and GPS height above sea level. During a drive to work.

• Pressure sensor: Bosch BME280
• GPS receiver: FGPMMOPA6H, MediaTek GPS Chipset MT3339

• Data sample:

  Sep 26 07:05:37 2017;0.0;19.65;66.62;1022.63;-11.1
  Sep 26 07:05:39 2017;0.02;19.64;66.66;1022.63;-11.1
  Sep 26 07:05:41 2017;0.02;19.64;66.54;1022.63;-11.1
  Sep 26 07:05:43 2017;0.04;19.64;66.63;1022.63;-11.1
  Sep 26 07:05:45 2017;0.06;19.63;66.9;1022.63;-11.1
  Sep 26 07:05:47 2017;0.05;19.63;66.67;1022.63;-11.1
  Sep 26 07:05:49 2017;0.03;19.62;66.87;1022.63;-11.1
  Sep 26 07:05:51 2017;0.03;19.62;66.67;1022.63;-11.1
  Sep 26 07:05:53 2017;0.04;19.62;66.64;1022.63;-11.1
  Sep 26 07:05:55 2017;0.03;19.62;66.66;1022.63;-11.1
  <snip>

• Gnuplot script:

  set title "BME280 - barometric height" 
  
  set term pngcairo enhanced font "arial,10" fontscale 1.0 size 1000,500
  set output "hoogte.png"
  
  set key left top
  set key box
  
  set autoscale 
  set grid back lw 1
  
  set datafile separator ";"
  # Tue Sep 26 07:05:37 2017;0.0;19.65;66.62;1022.63
  # Sep 26 07:05:37 2017;0.0;19.65;66.62;1022.63
  set datafile missing "?"
  
  set xdata time
  set timefmt "%b %d %H:%M:%S %Y"
  set format x "%T"
  set xtics  rotate by 90 offset 0,0 right
  
  set ylabel "meter above ground from start"
  set ytics 5
  
  set style line 1 lw 2 lc rgb "blue"
  set style line 2 lw 1 lc rgb "#708090"
  
  set label "< Brigde" at "Sep 26 07:13:23 2017", 12 tc rgb "black" left front offset 1,1
  set label "< Tunnel" at "Sep 26 07:33:30 2017", -20 tc rgb "black" left front
  set label "< Parking garage" at "Sep 26 07:37:00 2017", 3 tc rgb "black" left front
  set label "< The stairs up" at "Sep 26 07:41:00 2017", -15 tc rgb "black" left front
  set label "< Elevator to the 19th" at "Sep 26 07:43:30 2017", 3 tc rgb "black" left front
  
  
  plot "bme-en-gps.csv" using 1:2 title 'AGL barometric' with lines ls 1, \
       "bme-en-gps.csv" using 1:($6) title 'ASL from GPS' with lines ls 2