Importing the data into DISTANCE: transects with no observations

[Margherita Corti]

Hi everybody,

I have a problem with the import of data into DISTANCE 6.2 and density estimate. In my surveys I have some transects where I saw NO animals. How I can import them?
I try with a file that contain a row with the names and the regions for these transects with perpendicular distance and cluster size columns left blank as I saw in a tutorial.
However the program tells me that (in few words) it can't estimate density bacause I have blank columns.

How I can fix the problem?

Thanks to all,
M.

[Eric Rexstad]

Margherita

If you have no sightings on a transect, the information about that transect (its ID and its length) must still be imported into Distance--as you say it is only the perpendicular distance and cluster size that can remain blank.  However placeholders must be left for those blank fields. 

Here is an example

To import a data file into Distance, it must be in "flat file format" - i.e., arranged in rows and columns with one row for each observation. The actual number of columns depends on the type of survey (see later). Here's an example of part of a data file, from a line transect survey with two strata:

Stratum 1;100;Line 1;10;14
Stratum 1;100;Line 1;10;8
Stratum 1;100;Line 1;10;22
Stratum 1;100;Line 2;10.3;7
Stratum 1;100;Line 2;10.3;37
Stratum 1;100;Line 2;10.3;13
Stratum 2;123;Line 1;5.7;
Stratum 2;123;Line 2;8.4;27
Stratum 2;123;Line 2;8.4;76
Stratum 2;123;Line 2;8.4;44
Stratum 2;123;Line 2;8.4;7

In this file, the columns are separated by semicolons. Column 1 is the stratum name, column 2 is the stratum area, column 3 is the transect name, column 4 is the transect length, and column 5 is the perpendicular distance. Notice that all transects from the same stratum are grouped together on adjacent lines, and all observations from the same transect are grouped together. Notice also that the record "Line 1" in "Stratum 2" has no distance in the final column - this is a transect where no objects were seen.

This example does not have cluster size, simply perpendicular distance.  If there was cluster size, you would have two semicolons in succession for Stratum 2, Line 1 where there were no detections.

If this still does not answer your question, send me your data (off list) and I will look at it.

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-- 
Eric Rexstad
Research Unit for Wildlife Population Assessment
Centre for Research into Ecological and Environmental Modelling
University of St. Andrews
St. Andrews Scotland KY16 9LZ
+44 (0)1334 461833
The University of St Andrews is a charity registered in Scotland : No SC013532

[Serge Wich]

I was having some problems in distance 6.2 with the transects without sigthings. Left the perpendicular distance empty but then the analyses hung. In R with the same data in the Distance package the analyses also did not see to run. I am therefore not sure what is best to do. Can anyone provide suggestions on how to deal with this?

[Eric Rexstad]

Serge

I suppose it depends on *how* you left the distance field empty. Here
is an old webpage giving an example of a file to import, where one of
the transects contained no sightings:

http://creem2.st-andrews.ac.uk/preparing-your-data-for-use-in-distance-august-2015/

An example of a dataset for importing into R for use by the ds()
function can be found at this page, under the heading "Getting Started"

http://distancesampling.org/R/index.html

------
Eric Rexstad

Centre for Research into Ecological and Environmental Modelling

The Observatory, Buchanan Gardens
University of St Andrews

+44 (0)1334 461833
The University of St Andrews is a charity registered in Scotland : No
SC013532

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