Currently, there are 8 functions associated with the
`calculate`

verb in the `sgsR`

package:

`calculate_representation()`

- compare the representation of strata in existing samples`calculate_distance()`

- per pixel distance to the closest`access`

vector`calculate_pcomp()`

- principal components of the input`mraster`

`calculate_sampsize()`

- determines appropriate estimated sample sizes based on relative standard error`calculate_allocation()`

- sample allocation algorithm - proportional / optimal / equal / manual sampling`calculate_coobs()`

- determine how`existing`

samples are distributed among`mraster`

covariates`calculate_pop()`

- generate population level information (PCA / quantile matrix / covariance matrix) of input`mraster`

`calculate_lhsOpt()`

- testing to determine optimal Latin hypercube sampling parameters including sample number

`calculate_*`

functions serve as intermediary helper
functions. In this section we outline and demonstrate how these
functions can be used.

`calculate_representation()`

`calculate_representation()`

function allows the users to
verify how well the stratification is represented in their
`existing`

sample networks. Users input an
`sraster`

and their `existing`

samples to the
`calculate_representation()`

function, which will result in
tabular and graphical (if `plot = TRUE`

) outputs that compare
strata coverage frequency and sampling frequency.

```
#--- quantile sraster ---#
<- strat_quantiles(mraster = mraster$zq90,
quantiles nStrata = 8)
#--- random samples ---#
<- sample_srs(raster = sraster,
srs nSamp = 50)
#--- calculate representation ---#
calculate_representation(sraster = quantiles,
existing = srs,
plot = TRUE)
```

```
#> # A tibble: 8 × 6
#> strata srasterFreq sampleFreq diffFreq nSamp need
#> <dbl> <dbl> <dbl> <dbl> <int> <dbl>
#> 1 1 0.13 0.18 0.05 9 -2
#> 2 2 0.13 0.12 -0.0100 6 1
#> 3 3 0.13 0.1 -0.03 5 2
#> 4 4 0.12 0.06 -0.06 3 3
#> 5 5 0.12 0.16 0.04 8 -2
#> 6 6 0.12 0.1 -0.0200 5 1
#> 7 7 0.12 0.1 -0.0200 5 1
#> 8 8 0.12 0.18 0.06 9 -3
```

The tabular output presents the frequency of coverage for each strata
(`srasterFreq`

) (what % of the landscape does the strata
cover) and the sampling frequency within each strata
(`sampleFreq`

) (what % of total `existing`

samples
are in the strata). The difference (`diffFreq`

) between
coverage frequency and sampling frequency determines whether the values
are over-represented (positive numbers) or under-represented (negative
numbers). This value translates to a discrete `need`

attribute that defines whether there is a need to add or remove samples
to meet the number of samples necessary to be considered representative
of the strata inputted in `sraster`

.

Performing the algorithm on a sample set derived using
`sample_strat()`

exhibits proportional sampling to strata
coverage.

```
calculate_representation(sraster = sraster,
existing = existing,
plot = TRUE)
```

```
#> # A tibble: 4 × 6
#> strata srasterFreq sampleFreq diffFreq nSamp need
#> <dbl> <dbl> <dbl> <dbl> <int> <dbl>
#> 1 1 0.25 0.25 0 50 0
#> 2 2 0.25 0.25 0 50 0
#> 3 3 0.25 0.25 0 50 0
#> 4 4 0.25 0.25 0 50 0
```

Presence of very small (negligible) differences between
`srasterFreq`

and `sampleFreq`

is common.
**In these situations, it is important for the user to determine
whether to add or remove the samples.**

`calculate_distance`

`calculate_distance()`

function takes the input
`raster`

and `access`

data and outputs the per
pixel distance to the nearest access point. This function has a specific
value for constraining the sampling protocols, such as the
`sample_clhs()`

function, where the output raster layer can
be used as the `cost`

for the constraint. The output raster
consists of the input appended with the calculated distance layer
(`dist2access`

).

```
calculate_distance(raster = sraster, # input
access = access, # define access road network
plot = TRUE) # plot
```

```
#> class : SpatRaster
#> dimensions : 277, 373, 2 (nrow, ncol, nlyr)
#> resolution : 20, 20 (x, y)
#> extent : 431100, 438560, 5337700, 5343240 (xmin, xmax, ymin, ymax)
#> coord. ref. : UTM Zone 17, Northern Hemisphere
#> sources : memory
#> memory
#> names : strata, dist2access
#> min values : 1, 6.621213e-03
#> max values : 4, 1.061660e+03
```

*Access network polygons with a large number of features and/or
large spatial extent could result in slow processing times.*

`calculate_pcomp`

`calculate_pcomp()`

function takes `mraster`

as
the input and performs principal component analysis. The number of
components defined by the `nComp`

parameter specifies the
number of components that will be rasterized onto the output.

```
calculate_pcomp(mraster = mraster, # input
nComp = 3, # number of components to output
plot = TRUE, # plot
details = TRUE) # details about the principal component analysis appended
```

```
#> $pca
#> Standard deviations (1, .., p=3):
#> [1] 1.5479878 0.7359109 0.2493371
#>
#> Rotation (n x k) = (3 x 3):
#> PC1 PC2 PC3
#> zq90 0.6286296 -0.1795433 -0.7566961
#> pzabove2 0.5104140 0.8293596 0.2272450
#> zsd 0.5867729 -0.5290812 0.6130014
#>
#> $raster
#> class : SpatRaster
#> dimensions : 277, 373, 3 (nrow, ncol, nlyr)
#> resolution : 20, 20 (x, y)
#> extent : 431100, 438560, 5337700, 5343240 (xmin, xmax, ymin, ymax)
#> coord. ref. : UTM Zone 17, Northern Hemisphere
#> sources : memory
#> memory
#> memory
#> names : PC1, PC2, PC3
#> min values : -4.402269, -5.357801, -1.446156
#> max values : 5.282663, 2.155242, 1.510955
```

`calculate_sampsize`

`calculate_sampsize()`

function allows the user to
estimate an appropriate sample size using the relative standard error
(`rse`

) of input metrics. If the input `mraster`

contains multiple layers, the sample sizes will be determined for all
layers. If `plot = TRUE`

and `rse`

is defined, a
sequence of `rse`

values will be visualized with the
indicators and the values for the matching sample size.

```
#--- determine sample size based on relative standard error (rse) of 1% ---#
calculate_sampsize(mraster = mraster,
rse = 0.01)
#> nSamp rse var
#> 1 1394 0.01 zq90
#> 2 1341 0.01 pzabove2
#> 3 1859 0.01 zsd
```

```
#--- change default threshold sequence values ---#
#--- if increment and rse are not divisible the closest value will be taken ---#
<- calculate_sampsize(mraster = mraster,
p rse = 0.025,
start = 0.01,
end = 0.08,
increment = 0.01,
plot = TRUE)
#> 'rse' not perfectly divisible by 'increment'. Selecting closest sample size (rse = 0.03) based on values.
p#> $nSamp
#> # A tibble: 3 × 3
#> # Groups: var [3]
#> nSamp rse var
#> <dbl> <dbl> <chr>
#> 1 157 0.03 zq90
#> 2 151 0.03 pzabove2
#> 3 211 0.03 zsd
#>
#> $plot
```

`calculate_allocation`

`calculate_allocation()`

function determines how to
allocate samples based on the desired number of samples
(`nSamp`

) and the input `sraster`

. This function
is used internally in a number of functions, including `sample_strat`

. Currently, there are three
methods for allocations included: proportional (`prop`

;
default), optimal (`optim`

), equal (`equal`

), and
manual (`manual`

) allocation.

- Proportional - Samples are allocated based on the coverage area of the strata. This is the default allocation method.
- Optimal - Samples are allocated based on the variation within the strata.
- Equal - Same number of samples ( defined by
`nSamp`

) are allocated to each strata. - Manual - Numeric vector
`weights`

is provided allowing users to manually assign weights to strata.

```
#--- perform grid sampling ---#
calculate_allocation(sraster = sraster,
nSamp = 200)
#> strata total
#> 1 1 50
#> 2 2 50
#> 3 3 50
#> 4 4 50
```

```
#--- calculate existing samples to include ---#
<- extract_strata(sraster = sraster,
e.sr existing = existing)
calculate_allocation(sraster = sraster,
nSamp = 200,
existing = e.sr)
#> strata total need
#> 1 1 0 50
#> 2 2 0 50
#> 3 3 0 50
#> 4 4 0 50
```

Notice that some of the values under `total`

from the
result above are negative. The negative value indicates that the
`existing`

samples over represent those strata and that some
of the samples could removed to prevent over-representation.
`$total`

indicates the number of samples that could be added
or removed.

Optimal allocation method uses the variation within the strata metric
to allocate samples. This means that in addition to providing and
`sraster`

, that a specific metric (`mraster`

) must
be provided to calculate variation to optimally allocate samples.

```
calculate_allocation(sraster = sraster, # stratified raster
nSamp = 200, # desired sample number
existing = e.sr, #existing samples
allocation = "optim", # optimal allocation
mraster = mraster$zq90, # metric raster
force = TRUE) # force nSamp number
#> # A tibble: 4 × 3
#> # Rowwise:
#> strata total need
#> <dbl> <dbl> <dbl>
#> 1 1 27 77
#> 2 2 -14 36
#> 3 3 -26 25
#> 4 4 13 63
```

There may be situations where the user wants to have the same number
of samples allocated to each strata. In these situations use
`allocation = equal`

. In this case, `nSamp`

refers
to the total number of samples per strata, instead of the overall total
number of samples.

```
calculate_allocation(sraster = sraster, # stratified raster
nSamp = 20, # desired sample number
allocation = "equal") # optimal allocation
#> Implementing equal allocation of samples.
#> # A tibble: 4 × 2
#> strata total
#> <dbl> <dbl>
#> 1 1 20
#> 2 2 20
#> 3 3 20
#> 4 4 20
```

The code in the demonstration above yields a total of 80 samples (20
`nSamp`

for each of the 4 strata in
`sraster`

).

The user may wish to manually assign weights to strata. In this case,
`allocation = manual`

can be used and `weights`

must be provided as a numeric vector
(e.g. `weights = c(0.2, 0.2, 0.2, 0.4)`

where
`sum(weights) == 1`

). In this case, `nSamp`

will
be allocated based on `weights`

.

```
<- c(0.2, 0.2, 0.2, 0.4)
weights
calculate_allocation(sraster = sraster, # stratified raster
nSamp = 20, # desired sample number
allocation = "manual", # manual allocation
weights = weights) # weights adding to 1
#> Implementing allocation of samples based on user-defined weights.
#> strata total
#> 1 1 4
#> 2 2 4
#> 3 3 4
#> 4 4 8
```

The code in the demonstration above yields a total of 20 samples with
plots being allocated based on the `weights`

provided in
ascending strata order.

The following algorithms were initially developed by Dr. Brendan Malone from the University of Sydney. Dr. Brendan Malone and his colleagues graciously supplied an in depth description of the functionality of these algorithms, which were originally developed to improve soil sampling strategies. These functions were modified and implemented to be used for structurally guided sampling approaches. Many thanks to Dr. Malone for his excellent collaboration and being a proponent of open source algorithms.

Please consult the original reference for these scripts and ideas as their paper holds extremely helpful and valuable information to understand their rationale for sampling and algorithm development.

*Malone BP, Minansy B, Brungard C. 2019. Some methods to improve
the utility of conditioned Latin hypercube sampling. PeerJ 7:e6451 DOI
10.7717/peerj.6451*

`calculate_coobs`

`calculate_coobs()`

function performs the COunt of
OBServations (coobs) algorithm using `existing`

sample data
and `mraster`

covariates. This algorithm helps the user
understand how the `existing`

sample data set is distributed
among the landscape in relation to the `mraster`

covariates.
The output coobs raster can be used to constrain clhs sampling using the
`sample_clhs()`

function to the areas that are
under-represented.

The coobs raster determines how many observations are similar in terms of the covariate space at every pixel. This function takes advantage of parallel processing routines.

```
calculate_coobs(mraster = mraster, # input
existing = existing, # existing samples
cores = 4, # parallel cores to use
details = TRUE, # provide details from algorithm output
plot = TRUE) # plot
```

The following 2 algorithms presents the means to maximize the effectiveness of the latin hypercube sampling protocols.

`calculate_pop`

`calculate_pop()`

function calculates population level
statistics of the `mraster`

covariates that are being used,
which includes calculating the principal components, quantile &
covariate distributions, and Kullback-Leibler divergence testing. The
outputs produced from this functions are required to use the
`calculate_lhsOpt()`

function described in the following
section. Additionally, this algorithm can be pre-emptively used to
calculate `matQ`

and `MatCov`

, two values that are
used for the `sample_ahels()`

function. This will save
processing time during sampling.

```
#--- by default all statistical data are calculated ---#
calculate_pop(mraster = mraster) # input
```

The output list contains the following:

`$values`

- Pixel values from`mraster`

`$pcaLoad`

- PCA loadings`$matQ`

- Quantile matrix`$matCov`

- Covariate matrix

```
#--- statistical analyses can be chosen by setting their parameter to `FALSE` ---#
<- calculate_pop(mraster = mraster, # input
mat nQuant = 10) # desired number of quantiles
#--- use matrix output within sample ahels algorithm ---#
sample_ahels(mraster = mraster,
existing = existing,
nQuant = 10,
nSamp = 50,
matCov = mat)
```

`calculate_lhsOpt`

`calculate_lhsOpt()`

function performs a bootstrapped
latin hypercube sampling approach where population level analysis of
`mraster`

data is performed to determine the optimal latin
hypercube sample size.

Using data calculated using the `calculate_pop()`

and
varying sample sizes defined by `minSamp`

,
`maxSamp`

, `step`

and `rep`

. Sampling
protocols are conducted and statistical effectiveness of those sampling
outcomes are evaluated to determine where sample size is minimized and
statistical representation is maximized.

```
#--- calculate lhsPop details ---#
<- calculate_pop(mraster = mr)
poplhs
calculate_lhsOpt(popLHS = poplhs)
```

```
calculate_lhsOpt(popLHS = poplhs,
PCA = FALSE,
iter = 200)
```