[ADMB Users] Using asin with df1b2variables

[Ben Stevenson]

Thanks for the replies.

Dave --

We model the bearings using a von Mises distribution. We expect the animal
to keep on goin' in the same direction it did for the last point in time,
and then estimate the concentration parameter of the von Mises
distribution, which tells us how much it likes to deviate from this. So the
bearings go into the von Mises density function.

Johnoel -- Great, thanks!

Cheers,

Ben


On 17 October 2013 01:44, Johnoel Ancheta <johnoel at hawaii.edu> wrote:

> Thank you Ben for spotting the error.  It have been corrected in revision
> 1232 of the ADMB
> subversion repository.
>
> Johnoel
>
>
> On Wed, Oct 16, 2013 at 10:34 AM, Ben Stevenson <bcs5 at st-andrews.ac.uk>wrote:
>
>> Hi ADMBers,
>>
>> A couple of points:
>>
>> 1)
>> I have discovered that the function asin does not work with
>> df1b2variables, despite appearing in src/df1b2separable/df1b2f26. I am not
>> very familiar with the ADMB source code (or with C++ for that matter), but
>> I added asin to df1b2fun.h, rebuilt the source and everything worked fine.
>> Assuming I did the right thing, could someone with write access make the
>> same change to the subversion repo? Note that atan does appear but acos
>> does not.
>>
>> 2)
>> My reason for using asin was to calculate a bearing between two
>> locations, which are being treated as latent variables (and so modelled
>> with a random_effects_matrix). The model is an animal movement model, and
>> it is necessary to work out the direction the animal moved in from one time
>> point to the next. The problem for me was that the range of the function
>> asin is (-pi, pi) and obviously the range of the directions an animal can
>> move is (0, 2*pi). So it was necessary to translate from the first to the
>> second, but this depends on what quadrant the bearing belongs to (i.e., if
>> the animal moved northeast, southeast, southwest, or northwest). This is
>> because for each value returned in (-pi, pi) by asin, there are two
>> possible bearings in (0, 2*pi) that this could correspond to.
>>
>> "x_diff" and "y_diff" are the differences between the current and
>> previous x and y-coordinates, and "dist" is the distance between the
>> current and previous locations. I wanted to write something like:
>> bearing = asin(x_diff/dist)
>> if (southeast) bearing += M_PI  - 2*bearing;
>> if (southwest) bearing += M_PI - 2*bearing;
>> if (northwest) bearing += 2*M_PI;
>>
>> But this would break all sorts of ADMB rules with the if statements.
>>
>> In the end, I got around the problem by first emulating the performance
>> of R's sign() function (is there a way to do this in ADMB already??):
>> sign_x_diff = x_diff/fabs(x_diff);
>> sign_y_diff = y_diff/fabs(y_diff);
>>
>> And then creating some dummy variables that indicated the animal's
>> direction:
>> se_dummy = (sign_x_diff - sign_y_diff)*(sign_x_diff +
>> square(sign_y_diff))/4;
>> nw_dummy = (sign_y_diff - sign_x_diff)*(sign_y_diff +
>> square(sign_x_diff))/4;
>> sw_dummy = (-sign_x_diff - sign_y_diff)*(square(sign_x_diff) -
>> sign_y_diff)/4;
>>
>> Before using these instead of the if statements above to make the correct
>> adjustments to what was returned by asin:
>> bearing = asin(x_diff/dist);
>> bearing += se_dummy*M_PI - 2*bearing);
>> bearing += nw_dummy*2.0*M_PI;
>> bearing += sw_dummy*(M_PI - 2*bearing);
>>
>> Everything worked fine once I did all this, but it was quite a big
>> procedure to do something quite simple, and I was wondering if I was
>> missing an obvious solution I could use in future instead of the mess that
>> I've tried to explain above.
>>
>> Thanks in advance,
>>
>> Ben
>>
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>>
>>
>
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