To the list of courses || RASA2018 || To the theme || Estonian

Exercise 3968. Points 2, theme: Thinning

Open exercise
The attached file contains GPS-recorded trackpoints of Potentilla fruticosa field observations in a square kilometer. Code 1 marks the species presence locations, 0 marks the species absence. The aim is to remove close locations because these are duplicating each other and automatically tracked points close to find sites do not represent the species absence.
  1. Present a drawing depicting observation locations having interval at least 50 m. Condition: the observed presence locations must be preferred to absence locations while thinning. Add caption and legend of used signs to the drawing.
  2. How many present locations and absent locations remained after thinning?
Different data were used before July, 2019.
Data: Maranavaatlused.txt

Instructions

To get the correct result in the SDC, you should remove the excess points in the following order.
1. Remove track points within 50 m around find sites to assure maximum number of find sites.
2. Thinning retained track points to interval >50 m.
3. Thinning find sites to interval >50 m.
The calculator is removing locations at the given distance or closer. If the Clearance radius is empty or 0, only excess points with exactly matching coordinates will be removed.

Follow the instructions below.
  • Select thinning in the SDC.
  • Have a look on possible input data formats. It is easy to use the 2D format [X] [Y] [code] as the data are in this format.
  • Select One sample and copy data to the input cell. Write 50 as the clearance radius (the coordinates are in metres).
  • Check Output retained points only (see attached figure).
  • Press Calculate.
  • The result contains retained track points. GPS-recorded track points closer than 50 m from find sites are removed as not fitting to be exemplars of the species absence. Unfortunately the retained track points are too close to each other. Close locations are redundant; these cannot be considered as independent exemplar locations.
  • Copy the retained track points to a file using the link copy to clipboardin the header of results panel.
  • Select One sample in the calculator. Clear the inpun cell. Copy the saved track points to the input cell and launch thinning to interval at least 50 m. Save the thinned track points. These are the selected exemplars of the species absence sites.
  • Next, clear the input cell and copy the species find sites to the input. Thin the find sites to interval at least 50 m; save the thinned find sites.
  • Finally, you should make a chart indicating the location of retained points. A simple drawing can be created according to the following procedures: 1) select Two samples in the calculator; 2) copy the thinned find sites to one input cell and the thinned absent locations to another. Press XY plot. Save the figure, overwrite the legend or specify the meaning of signs in the figure caption.

Such thinning is used for example in the following studies:
Remm K., Linder M., Remm L. 2009. Relative density of finds for assessing similarity-based maps of orchid occurrence. Ecological Modelling 220 (3), 294-309, doi: 10.1016/j.ecolmodel.2008.10.017.
Remm K., Remm L. 2009. Similarity-based large-scale distribution mapping of orchids. Biodiversity and Conservation 18 (6), 1629-1647, doi: 10.1016/10.1007/s10531-008-9547-5.
Remm K. 2016. Selecting site characteristics at different spatial and thematic scales for shrubby cinquefoil (Potentilla fruticosa L.) distribution mapping. Forestry Studies | Metsanduslikud Uurimused 64, 17–38. doi:10.1515/fsmu-2016-0002.
Remm K., Remm L. 2017. Shrubby cinquefoil (Dasiphora fruticosa (L.) Rydb.) mapping in Northwestern Estonia based upon site similarities. BMC Ecology 2017 Feb 21;17(1):7. doi: 10.1186/s12898-017-0117-0.
Log in to send your results and to see the expected answer and responses from other students.