How many stoats or weasels did the trap line catch last year?

Data Analysis in QGIS

• Selecting vector layer attributes• Create a frequency layer• Display frequency using the symbology menu

Select the layer with trap data Right click to find the ‘attributes’.

Look for ‘stoat’ in the ‘catch’ attributes, and ‘Search’

You can “Show selected records only”

Save selection of stoats as a new file

Right-click layer, and ‘Save Selection As’.

In order to see the location and frequency of stoats caught, you need to map the number of stoat-catch events.

This is done by using your stoat-catch data layer, and displaying the number of stoats caught in each trap.

1. Create a polygon of the trap locations, by making a buffer around each trap point. Buffer = 5m

Where were most stoats caught?

2. Use ‘Points in Polygon’ tool to find the frequency of stoat records at each trap site. (PNTCNT = number of stoats/trap)

The ‘Stoat_catch’ point layer provides the stoat catch data. The ‘TraplinePoint_buff’ is the polygon of trap sites.

…. now to display the frequency, not just a square symbol ….

Use the layer attribute, column PNTCNT, for the frequency.

Change the symbol to ‘graduated’. Change the symbol marker type, the colour, and the size.

Repeat for each of the graduated symbols to distinguish them by both colour and size. The traps where no stoats were seen is transparent, to show ‘zero’. Check that the labels are correct, like 1, 2 or 3, and edit if necessary (Not decimal).

Stoats – frequency of trap catch

Weasels - frequency of trap catch

These GIS layers and others are from the ‘Taranaki dataset’ and may be downloaded from the Drop Box folder.

This data is used in demonstrations for the ‘Mapping and Analysis Tools’ module.

Prepared by MAIN Trust NZ and DataMap Ltd., 2013