Figure 5
========
The chart in Figure 5 is derived from a subset of the total dataset. For each sensor,
approximately 8192 rows were selected to plot the graph. The 8192 figure was chosen
(somewhat arbitrarily) to ensure a plot with sufficient horizontal resolution for
a reasonable looking plot. The rows used to plot data for each sensor were selected
using a SQL query similar to:-

  SET @a := 0;
  SET @rows = (SELECT COUNT(*) FROM `Anchorage-data` WHERE `node` = 1 AND `time` >= "2007-01-01" AND `time` <= "2019-01-15");
  SET @n = FLOOR(@rows / 8192);
  SELECT * FROM (SELECT `time`, `temp1`, `humid1`, @a:=@a+1 AS rc FROM `Anchorage-data` WHERE `node` = 1 AND `time` >= "2007-01-01" AND `time` <= "2019-01-15") rc WHERE rc % @n = 0;



The data in the dataset are raw data from the sensor. In order to convert these
data into temperature or relative humidity readings, the following formulae are
used:-

  Temperature (Celsius) =  (-39.6 + 0.01* raw_temperature)

  Relative Humidity (%) =  (Temperature - 25.0)
                          * (0.01 + 0.00008 * raw_humidity)
                          - 4.0
                          + 0.0405 * raw_humidity
                          - 0.0000028 * raw_humidity * raw_humidity

  The above formulae are derived from the SHT11 datasheet [1].

Each sensor's data are listed in separate pages of the 'Figure-5-anchorage-data.ods'
spreadsheet file.

Where rows of data are more than two days apart, blank rows have been manually inserted
into the spreadsheet in order to ensure that the line plot is discontinuous between these
data points.

Outliers, and other sensor data that is clearly erroneous (usually correlated with a
depleting battery voltage) has been manually removed by cut and pasting these data
to another column in the same row.


Figure 6
========
The chart on Figure 6 is obtained using the following query:-

  SELECT DATE(time), COUNT(*) FROM `Anchorage-data` GROUP BY DATE(time);

the results of which are in the 'Data per day' page of the 'Figure-6-data-received-at-gateway.ods'
spreadsheet file.

An additional, page - 'Battery change' has been added in order to indicate on the chart
the dates on which sensors had batteries changed.


References
==========
[1] Sensirion SHT11 Datasheet
    https://web.archive.org/web/20201216134918/https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/2_Humidity_Sensors/Datasheets/Sensirion_Humidity_Sensors_SHT1x_Datasheet.pdf