**How many people are there really in a demonstration? **A question that most of us will ask when listening to the disparate figure of attendees according to who performs the calculation. The feeling, many times, is that these figures do not follow any methodology or lack scientific rigor.

Could we stick to the ‘accuracy’ of the figures? Could you estimate how many people participated in a demonstration?

The answer, fortunately, is affirmative. What we bring you today is a **gvSIG Desktop plugin that calculates the number of people attending a demonstration. Or, rather, that calculates the population in a certain area**. Exactly, the mathematical procedure to calculate the number of demonstrators is the same than the procedure used to calculate the number of plants of a certain species in a specific area (or people concentrated in an area, such as a certain cultural event).

With this new gvSIG Desktop add-on, in short, we can get an idea of the **people that there are in an area at a specific time **approximately.

The two fundamental variables for the calculation are the space occupied and the density of individuals per unit of area. By crossing both, we get results that can be very close to reality.

The first variable is simple to be obtained: it is basically to define the area or areas of study and that in gvSIG Desktop it is translated into a layer that contains the polygon or polygons that represent the space to be studied.

The second variable -the density- requires an observation work already. In the case of a demonstration, it will try to define (either in person or through photographs) the number of people per square meter.

**Let’s see an example** where we use the tool. For this we are going to suppose that a concentration has been called in the city of Valencia for gvSIG users.

Let’s imagine that we have a View (with EPSG: 3857) where we have loaded Mapnik (from OSM) as a background layer.

The first thing we will do is to draw the layer with the area occupied by the concentration, as shown in the video. When launching the ‘population calculator’, the first thing we will indicate is the layer that contains the surface and it will automatically give us the area in surface units, square meters in our case (in the ‘Properties’ of the View we can select other work units, for example miles).

The next step will be to define the density. That is, if we estimate that each square meter has been occupied by just one person, by two, four… In this sense we indicate that densities from four to six individuals per square meter are very high densities in the case of groups of people. Mass behaviour in public spaces studies indicate that more than six people per square meter reaches the collapse risk range. It is easy to visualize this issue if we imagine a 150 square meters house with more than 1,000 people inside it, which would be equivalent to a density of 7 people per square meter.

In our example we are going to assume that there are 4 people per square meter. When entering this data, it automatically indicates the total number of attendees to this fictitious ‘concentration of gvSIG users’. By changing the number of individuals per area unit, it automatically recalculates the total population. And if we modify the area we can recalculate the number of attendees.

This example can be seen in the following video:

The plugin that we have developed is really basic, a concept test, and could be extended, for example, by allowing calculations of different areas with different densities. In any case, it is a demonstration that this type of measurement is possible, and thanks to this plugin anyone can do it very easily and in a short time.

The plugin is available to be **installed from the ‘Add-ons Manager’**, ‘Installation from URL’ option, and looking for the plugin called ‘**Population Calculator**‘.

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