Wednesday, February 5, 2014

Creating Histogram

In statistics, a histogram is a graphical representation of the distribution of data. It is an estimate of the probability distribution of a continuous variable and was first introduced by Karl Pearson.

In this article i will be demonstrating how to use raw data from anemometer creating histogram.
This way histogram can be used to as an representation of selected wind speeds to total wind speed.

This data can be later used to calculate theoretical (and real) power output from current wind speeds or proposed wind turbine and also to estimate energy production.



We will start with data output from digital anemometer.
Usually data is available in .csv format to process the data you will need spreadsheet application, MS Excel of LibreOffice Calc.
In this article i will use LibreOffice Calc.

Note that processing data may vary and may depends how anemometer formats raw data.

Download sample raw data.

Open file with raw data. In my case LibreOffice Calc automatically recognize data and separates it (see Figure 1), in my experience older version of MS Excels don't do this and you need manual to set separation values.

Fig.1 Importing data to LibreOffice Calc
Now create new sheet to separate raw data from processed data. In rows add wind speeds (classes) that you want to be showed in histogram.

In this example i will use 0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5 

Now use Function Wizard chose Frequency function to create histogram, (Insert > Function or CTRL + F2).

Fig. 2 Using function wizard in LibreOffice Calc

As data use wind speeds from raw data. If your anemometer logs min, max and average speed use average wind speed.
For classes use wind speeds that we previously created.

Now we have frequency of selected wind speeds from raw data. To get percentage in column use formula:

=FREQUENCY / (NUM.DATA.ENTRY / 100)

Where:

FREQUENCY is number that we got from frequency function.
NUM.DATA.ENTRY is total number of data entries from raw data.

You will get table that look like this:

Table 1 Wind speeds, frequency and percentage

From data in table 1 we can create chart as show in figure 3.

Fig. 3 Histogram chart
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