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The following steps will explain the processes by which transects can be randomly or systematically laid for sampling.


Randomization of Line Transect Sampling -

For transects to be laid randomly the starting point and direction of the transect (bearing) have to be decided randomly. (The length of the transect is standardized for a study and already decided upon before laying the transects.) Thus any method that can achieve a high level of randomness for the selection of a starting point in a transect and the direction the transect will be laid will be satisfactory for this.


1.Computerized Methods -


Many geographic information system (GIS) softwares (such as ArcGIS or a free alternative  DIVA GIShave functions that can be used to randomly lay transects satellite images of the study area (which can be obtained freely from  many sources(a list will be put up soon). Once these are laid on the satellite image by the computer program their gps coordinates can be obtained and you can go to the site of those coordinates to lay the transect. This method does require a gps device, if that is not available the next method will have to suffice, though it is not as effective as this one in randomization.



2.Methods that can be used in the field -

Upon reaching the study site randomization of the selection of the location of the transect can be achieved up to an extent within a small area of the locality by having a researcher close their eyes and turn around repeatedly to lose track of their bearing and cast a stone. The location the stone is dropped at can serve as the starting point of the transect and the bearing can be determined by the same process of having a researcher close their eyes and turn around repeatedly to lose track of their bearing and eventually stop in a direction which becomes the bearing of the direction the transect will be laid in.


3. Using print outs of Google Earth Images -

If the study area is not too large and is well known to the researcher, a dart or other similar object can be thrown at a print out of a google earth image of the area. The location the dart falls in becomes the starting point of the transect and the bearing can be decided using the method given above.


Systematization of Line Transect Sampling-

For systematization of transect sampling, the starting points and end points of the transects have to be decided upon in a systematic manner. As above there are computerized and field based methods -


1. Computerized Methods -


A grid is created to overlay the satellite image of the study site (which may be found in a  variety of sources (list will be put up soon)using functions in a GIS software. The length of the transect chosen by the researcher can be the length of the side of the grid square. Thus the transects in the field would then lie between two adjacent grid square corners that would serve as the endpoints of the transect. Alternatively the transect could be the line connecting two diagonal corner points of each grid. The transects should also be laid out in a manner that will make it easy to sample them in the field. In the diagram below the direction of each transect differs but this makes it easy to walk all the three transects in the top three grid squares without losing time/energy/money moving from the end point of one transect to the start of the next (which is what would have to be done if all the transects pointed in the same direction). The lower row has not been optimized in the same manner with regards to the upper row (i.e from the end point of the last transect on the first row, a researcher will have to spend time/energy/money moving to the beginning of the transects on the second row), this is done so that the researcher does not end up too close to areas which have already been sampled before as in the case of studies on relatively larger or mobile animals like birds, the researcher may end up counting the same animals in the north of the lower row that moved down from the south of the upper row. Another way to prevent this is to leave enough space i.e one or more rows of grid squares that you don’t sample lying between rows which have transects and that you do sample.



systematically laying line transects



2. Methods that can be used in the field -

After reaching the field site a transect point can be laid at regular intervals of a certain distance (like 1km for example) and the bearing of each transect can be the same. Though a device would be required to accurately obtain distances traveled when the intervals are far apart.


About the Author of this Article


Southwood T. , and P. Lenderson. Ecological Methods - Published by Blackwell Science Ltd.