# Optimization tips

### achieving robust location estimates

#### Koen Hufkens

# 1 Introduction

Geolocation by light is as much an art as it is a science, as certain decisions in your data workflow will affect the accuracy of your location estimates. It is therefore key to understand which decisions to be mindful of during your workflow. Below I’ll briefly touch upon some of these aspects.

# 2 Pre-processing

Before you start an estimation location there are a number of steps you can undertake to maximize the quality of your results.

## 2.1 Data quality

Invariably the quality of your location estimates will depend on the quality of your input (logger) data. This means that poor quality data (due to false twilights or nest visits during the day) will negatively affect a location estimate’s accuracy. To remove the most common sources of error the `stk_screen_twl()` function is included. Eliminating poor quality days will improve location estimates as there is a temporal dependency between the current estimate and the previous one.

To inspect data quality you can visualize the full light profile by using the `stk_profile()` function.

    # an overview profile
    cc876 |> stk_profile()

You can also use the `stk_filter()`, used to flag and filter the data to process using the `range` parameter, with the `plot` parameter set to TRUE. This returns a plot of the selected data as well as the annotated dataset.

    # an overview profile
    cc876 |> stk_filter(range = c(0.09, 400), plot = TRUE)
    #>
    #> ── Filtering data ──────────────────────────────────────────────────────────────
    #>

    #> # A tibble: 8,640 × 9
    #>    logger date_time           date         hour measurement value offset
    #>    <chr>  <dttm>              <date>      <dbl> <chr>       <dbl>  <dbl>
    #>  1 CC876  2021-08-02 00:04:10 2021-08-02 0.0694 lux          0.08   24.0
    #>  2 CC876  2021-08-02 00:09:10 2021-08-02 0.153  lux          0.08   24.0
    #>  3 CC876  2021-08-02 00:14:10 2021-08-02 0.236  lux          0.08   24.0
    #>  4 CC876  2021-08-02 00:19:10 2021-08-02 0.319  lux          0.08   24.0
    #>  5 CC876  2021-08-02 00:24:10 2021-08-02 0.403  lux          0.08   24.0
    #>  6 CC876  2021-08-02 00:29:10 2021-08-02 0.486  lux          0.08   24.0
    #>  7 CC876  2021-08-02 00:34:10 2021-08-02 0.569  lux          0.08   24.0
    #>  8 CC876  2021-08-02 00:39:10 2021-08-02 0.653  lux          0.08   24.0
    #>  9 CC876  2021-08-02 00:44:10 2021-08-02 0.736  lux          0.08   24.0
    #> 10 CC876  2021-08-02 00:49:10 2021-08-02 0.819  lux          0.08   24.0
    #> # ℹ 8,630 more rows
    #> # ℹ 2 more variables: hour_centered <dbl>, selected <lgl>

## 2.2 Data frequency

Unlike a purely twilight based approach to location estimates the {skytrackr} package uses all, or part of, the measured diurnal light cycle. By adjusting the `range` parameter you can include more or less data. Depending on the quality of the data including more data, outside the strict twilight range can be beneficial. You can explore the influence of your range parameter using the `stk_filter()` function (see above).

Including more data will increase the required computational power, i.e. time, for a good estimate. It is also important to note that some loggers (e.g. those by the Swiss ornithological society) do not register a full diurnal profile. In short, always inspect a daily light profile to establish if a full diurnal cycle is recorded, to exclude any baseline and saturated values (i.e. fill values), and assess the quality (noise) of the data.

## 2.3 The scale parameter

The scale parameter sets the light loss of the sky illuminance in the skylight model using the `sky condition` parameter in order to account for environmental conditions. This parameter is estimated during optimization, but at times competes with the estimated latitude parameter (depending on the optimization setup and used data). This parameter therefore needs to be chosen carefully, to allow for enough flexibility to account for environmental factors.

These environmental factors take several forms. For an unobstructed sky view the scale parameters (sky condition) varies between values of one (1) for a clear sky, to 3 for an average sky (30% cloud cover) to ten (10) for a dense dark cloudy sky (uncommon). However, if an individual spends a lot of time in vegetation illuminance values can be lower as the environment further attenuates the ambient light conditions. Should a sensor be covered by additional feathers or fur the registered values might be lower still.

### 2.3.1 Light loss theory

Light loss can be described as:

light loss = \(Q_{F}(sky condition, LAI)\) + \(f_{sensor}(sky condition, coverage)\) + \(\epsilon\)

Where both \(Q_{F}()\) and \(f_{sensor}(coverage)\) are dependent on the initial sky conditions in their own right. During optimization the upper bound of the scale should however not be a mere guess. Depending on the behaviour of the individual a physics informed constraint can be used.

Where light loss through a canopy is described according to the Beer–Lambert Law.

\(Q_F\) = \(Q_0 e^{–kF}\)

Where \(Q_{F}\) is the light level for a given level F, with k dependent on the canopy complexity (where literature often cites a value of 0.5 as a good middle ground). Maximum F is determined by the density of the vegetation or Leaf Area Index (LAI, dimensionless), expressed as leaves (\(m^2\)) per surface area (\(m^2\)). LAI reach a maximum of 10 for certain dense needleleaf forests, but are generally far lower.

Assuming a k of 0.5 and a maximum LAI of 6 (a reasonable maximum for broadleaf forests) we get a light loss relative to the original solar illuminance of about 5%. We can map the `sky condition` parameter to LAI (using an average sky as a reference). This inverse mapping allows us to estimate the vegetation density in which an individual (bird) is located (assuming an average sky and limited sensor losses). Turning this around, if you know the ecology of an individual you can then use this relationship to set the upper scale factor, under ideal conditions. The below graph shows the relationship between the scale factor and LAI for an average sky (sky conditions of 3 in the {skylight} model). Working on the assumption that most dense vegetation have a LAI of four to six, rarely reaching 10, one can expect the scale factor to rarely exceed 50 (unless other factors are at play such as a dirty or partially covered sensor).

    # calculate the attenuation Beer-Lambert for LAI
    lai_range <- seq(0, 10, by = 0.1)
    lai_att <- 100 - (exp(-0.5 * lai_range) * 100)

    # set the range of scale factors
    scale_factor <- seq(1, 2000, by = 1)

    # calculate "sky condition" based attenuation values
    sky_att <- lapply(scale_factor, function(i){
      ideal <- skylight::skylight(
        date = as.POSIXct("2022-01-01 12:00:00"),
        latitude = 0,
        longitude = 0,
        sky_condition = 3
      )$sun_illuminance

      c <- skylight::skylight(
        date = as.POSIXct("2022-01-01 12:00:00"),
        latitude = 0,
        longitude = 0,
        sky_condition = i
      )$sun_illuminance / ideal

      c <- c * 100
      c <- ifelse(c > 100, NA, c)
      c <- 100 - c
    }) |> unlist()

      lai_scale_factor <- lapply(lai_att, function(a){
        scale_factor[which.min(abs(sky_att - a))]
      }) |> unlist()

    ggplot2::ggplot() +
      ggplot2::geom_line(
        aes(
        lai_range,
        lai_scale_factor
        )
      ) +
      ggplot2::labs(
        title = "LAI ~ scale factors",
        x = "LAI",
        y = "scale factor"
      ) +
      ggplot2::theme_minimal()

### 2.3.2 Estimating the scale from data

Working on the assumption that the sensor is correctly installed and the sensor error is minimal we can estimate the light loss from the data itself if a full diurnal profile is measured. Here, the difference between the maximum (global) solar illuminance at noon can be compared with the measured maximum daily values.

These values should give a crude estimate of the total light loss (without a need for attribution to vegetation, sky or other environmental conditions). Generally, Migrate Technology Ltd. sensors track close to true physical values. The included function `stk_calibrate()` estimates daily scale factors across the data (and their range), which can then be used as upper value to the scale parameter in `skytrackr()`.

## 2.4 Optimization iterations

There is also a trade-off between the amount of data used in a location estimate and the number of iterations used during optimization. If your data quality, and/or frequency, is low it is advised to increase the number of optimization iterations. You can consult the Gelman-Rubin Diagnostic values for the quality of the fit (use the `stk_map()` function after a fit, or plot the optimization process).

For high quality data 3000 iterations generally yields good results, but increasing this number to 6000 might provide a more robust estimate. It is advised to inspect the performance of the routine on a single logger, before proceeding to (batch) process all data. Iteration values in excess of 10K should generally not be required.

## 2.5 Step-selection dynamics

The step-selection function constrains the validity of a proposed location estimate. However, the function used is approximate only. It must also be noted that while an individual might move a long distance across a day (in absolute sense), its position from day-to-day might not move much (e.g. in the most extreme case, there is no day-to-day movement if the individual returns to a nesting location). The step-selection function should reflect short-distance ranging movements (i.e. rapid decay) rather than long-distance migration movements.

## 2.6 The light model used

The package provides two modes of calculating a diurnal light profile, fit to the data. The normal, default, “diurnal” mode calculates light levels for a single (static) location (latitude, longitude), while the “individual” model calculates light levels along a rhumb line track between the last estimated position and a target (end) position. The speed depends on the track length, and is constant throughout. The “individual” model therefore corrects for the slight changes in light levels when moving with or against a changing twilight.

For example, with twilight at dusk approaching from the east, flying towards the east will shorten the actual twilight. The inverse is true when flying westward during dawn, prolonging twilight conditions. These effects are particularly important for fast east-west movements. The “individual” model influences both latitude and scale parameter components. Note that the increased complexity of the model influences parameter estimation and generally requires increasing the number of required iterations, and potentially the upper scale value.

# 3 Post-processing

After estimating locations you can inspect the location estimation data using the `stk_map()` function. This will give you an initial idea on the accuracy of the estimates. The spread of the uncertainty on both longitude and latitude parameters can help determine the quality of the estimated locations. If available, a Gelman-Rubin Diagnostic (or grd value) is returned in the data output. Gelman-Rubin Diagnostic values < 1.05 are generally considered showing convergence of the parameter (location) estimates. Spurious patterns, such as coordinates bouncing between two locations are also a telltale sign that your model was poorly constrained and converged on a faulty solution (see highlighted red sections in the panel plot below). In those cases you mostly either have to increase the scale upper bound, and/or increase the search space for your solution by increasing the number of iterations during optimization.

Figure 3.1: Overview plot after location estimation

> It is helpful to realize that models, fit to data, can be “right” for all the wrong reasons. Even when parameter values converge this does not guarantee an absolute “true” location. It is therefore important to realize that broad patterns should hold, but individual short sections of tracks can vary due to parameter settings.