Explanation on sensor_msgs/LaserScan and time

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Hi everyone !

I am currently working on LaserScan messages and I don't quite understand how time related attributes should be filled. If I look at the documentation :

Header header            # timestamp in the header is the acquisition time of 
                     # the first ray in the scan.
                     #
                     # in frame frame_id, angles are measured around 
                     # the positive Z axis (counterclockwise, if Z is up)
                     # with zero angle being forward along the x axis

float32 angle_min        # start angle of the scan [rad]
float32 angle_max        # end angle of the scan [rad]
float32 angle_increment  # angular distance between measurements [rad]

float32 time_increment   # time between measurements [seconds] - if your scanner
                         # is moving, this will be used in interpolating position
                         # of 3d points
float32 scan_time        # time between scans [seconds]

float32 range_min        # minimum range value [m]
float32 range_max        # maximum range value [m]

float32[] ranges         # range data [m] (Note: values < range_min or > range_max should be discarded)
float32[] intensities    # intensity data [device-specific units].  If your
                         # device does not provide intensities, please leave
                         # the array empty.

I understand that scan_time is the total duration of one scan (i.e. duration of my n measurements) and that time_increment is the duration of one measurement (so scan_time = time_increment x n).

But then, I began to play around with this bag file, which replay Laser Scan messages ( https://github.com/mlab-upenn/f1_10_c... ), and I realized that my assumption are not right. By doing :

rostopic echo /scan

I got one of the message:

header: 
  seq: 40906
  stamp: 
    secs: 1458504920
    nsecs: 148172000
  frame_id: "laser"
angle_min: -1.57079637051
angle_max: 1.56643295288
angle_increment: 0.00436332309619
time_increment: 1.73611115315e-05
scan_time: 0.0250000003725
range_min: 0.0230000000447
range_max: 60.0

I know I have n=720 measurements, because I watched for ranges size and because:

full_angle/angle_increment = (1.56643295288+1.57079637051) / 0.00436332309619 = 719 (+ 1 for the first element).

But when I do the same with time, I get 2 x n as result:

scan_time/time_increment = 0.0250000003725÷1.73611115315×10^05 = 1440

So why is that ? Am I missing something ?

This bag file seems correct, beacause I managed to reproduce the simulation as the author did in a video ( https://www.youtube.com/watch?v=3C_eR... ).