Measure walking speed with calibration

Tell CV-Scope how big the floor really is, with one known distance or four measured points, and read speeds in metres per second.

Time: 25 minutes
You need: The route scene from guide 11, and a tape measure for a real camera
Pages of the app: Scene Builder, Experiments, Data
Topics: Analysis

What you will be able to do

Calibrate with one known distance
Calibrate a perspective view with four reference points
Read speeds per track, route and event
Know the limits of each method

Without calibration, CV-Scope measures distance in fractions of the picture, and speed in fractions of the picture per second. A calibration maps the picture to the real floor, so distances come out in metres and speeds in metres per second.

There are two methods:

Method	What you enter	Good for
Known distance	One drawn line and its real length	Cameras looking almost straight down
Reference points	Four or more points on the floor, with their real positions	Any camera looking at a flat floor at an angle

Practice: one known distance

The corridor clip comes without measurements. For practice, pretend that the Entrance gate from guide 11 is 4 metres long.

Open the Scene Builder of the Corridor camera with the route scene.
Press V for the Select tool and click an empty spot of the picture, so that no shape is selected. The inspector shows the scene overview.
Scroll to Calibration (optional). Keep Unit as m.
In Known distance, choose Entrance. Replace the 5 in the box next to it with 4.
The note above changes to Scale-only calibration. Click Save in the top bar, or press Ctrl+S.
Click New experiment, name it Walking speed, keep Person and click Create experiment. Then click Start.
Open Results when the run has finished.

With the 4-metre assumption the sample gives these speeds:

Where	Value
Mean track speed at the top of the summary	about 1.7 m/s
`Route A`, Mean speed (m/s) in the routes table	about 2.4
`Route B`, Mean speed (m/s)	about 2.2

On the Events tab, the Speed column shows the mean speed of each route and zone visit.

The corridor camera looks at the floor at an angle, so one scale for the whole picture is only a rough approximation here. The next method is the right one for such a view.

A real camera: four reference points

You need four or more points on the floor that you can see in the picture and measure in the room. Corners of floor tiles, painted lines, or taped crosses work well.

Measure the points. Pick one corner of the area as the origin. Measure each point’s distance from it along two directions at right angles, in metres. Note them as X and Y.
Place the points. In the Scene Builder, press K for the Calibration tool. Click each point on the floor in the picture. Each click adds a row to the Reference points table in the inspector.
Enter the positions. Type each point’s Ground X and Ground Y into its row.
Check the note. With four or more points it reads Perspective calibration active. Save the scene.

Tips for good results:

Spread the points out. Place them near the corners of the area people walk through, not in a small cluster.
Stay on one plane. All points must lie on the same flat floor. A step or a ramp breaks the mapping.
Measure carefully. An error of 10 cm over 5 metres is a 2 % error in every speed.

What the speeds mean

Ground point. Speeds follow the bottom centre of each box, about where the feet are. A person partly hidden behind a counter has a box that ends higher up, so their position and speed are less reliable.
Routes. A route’s mean speed is the path length from start to end, divided by the time taken.
Checking. Draw a zone over a measured stretch of floor and walk through it at a steady pace while someone times you. The Speed of your zone visit on the Events tab should be close to the distance divided by the time.

Guide 17 checks how accurate the counts of a run are.