Visualizing IOT time series with QGIS & MobilityDB

Today’s post presents an experiment in modelling a common scenario in many IOT setups: time series of measurements at stationary sensors. The key idea I want to explore is to use MobilityDB’s temporal data types, in particular the tfloat_inst and tfloat_seq for instances and sequences of temporal float values, respectively.

For info on how to set up MobilityDB, please check my previous post.

Setting up our DB tables

As a toy example, let’s create two IOT devices (in table iot_devices) with three measurements each (in table iot_measurements) and join them to create the tfloat_seq (in table iot_joined):

CREATE TABLE iot_devices (
    id integer,
    geom geometry(Point, 4326)
);

INSERT INTO iot_devices (id, geom) VALUES
(1, ST_SetSRID(ST_MakePoint(1,1), 4326)),
(2, ST_SetSRID(ST_MakePoint(2,3), 4326));

CREATE TABLE iot_measurements (
    device_id integer,
    t timestamp,
    measurement float
);

INSERT INTO iot_measurements (device_id, t, measurement) VALUES
(1, '2022-10-01 12:00:00', 5.0),
(1, '2022-10-01 12:01:00', 6.0),
(1, '2022-10-01 12:02:00', 10.0),
(2, '2022-10-01 12:00:00', 9.0),
(2, '2022-10-01 12:01:00', 6.0),
(2, '2022-10-01 12:02:00', 1.5);

CREATE TABLE iot_joined AS
SELECT 
    dev.id, 
    dev.geom, 
    tfloat_seq(array_agg(
        tfloat_inst(m.measurement, m.t) ORDER BY t
    )) measurements
FROM iot_devices dev 
JOIN iot_measurements m
  ON dev.id = m.device_id
GROUP BY dev.id, dev.geom;

We can load the resulting layer in QGIS but QGIS won’t be happy about the measurements column because it does not recognize its data type:

Query layer with valueAtTimestamp

Instead, what we can do is create a query layer that fetches the measurement value at a specific timestamp:

SELECT id, geom, 
    valueAtTimestamp(measurements, '2022-10-01 12:02:00') 
FROM iot_joined

Which gives us a layer that QGIS is happy with:

Time for TemporalController

Now the tricky question is: how can we wire our query layer to the Temporal Controller so that we can control the timestamp and animate the layer?

I don’t have a GUI solution yet but here’s a way to do it with PyQGIS: whenever the Temporal Controller signal updateTemporalRange is emitted, our update_query_layer function gets the current time frame start time and replaces the datetime in the query layer’s data source with the current time:

l = iface.activeLayer()
tc = iface.mapCanvas().temporalController()

def update_query_layer():
    tct = tc.dateTimeRangeForFrameNumber(tc.currentFrameNumber()).begin().toPyDateTime()
    s = l.source()
    new = re.sub(r"(\d{4})-(\d{2})-(\d{2}) (\d{2}):(\d{2}):(\d{2})", str(tct), s)
    l.setDataSource(new, l.sourceName(), l.dataProvider().name())

tc.updateTemporalRange.connect(update_query_layer)

Future experiments will have to show how this approach performs on lager datasets but it’s exciting to see how MobilityDB’s temporal types may be visualized in QGIS without having to create tables/views that join a geometry to each and every individual measurement.

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