The aim of this Rpackage is to enable fast, accurate and user friendly calculation longitudinal steepness of linear features such as roads and rivers, based on commonly available input datasets such as road geometries and digital elevation model (DEM) datasets.

Install the development version from GitHub with:

```
# install.packages("remotes")
remotes::install_github("itsleeds/slopes")
```

Load the package in the usual way:

We will also load the `sf`

library:

The minimum data requirements for using the package are elevation points, either as a vector, a matrix or as a digital elevation model (DEM) encoded as a raster dataset. Typically you will also have a geographic object representing the roads or similar features. These two types of input data are represented in the code output and plot below.

```
# A raster dataset included in the package:
class(dem_lisbon_raster) # digital elevation model
#> [1] "RasterLayer"
#> attr(,"package")
#> [1] "raster"
summary(raster::values(dem_lisbon_raster)) # heights range from 0 to ~100m
#> Min. 1st Qu. Median Mean 3rd Qu. Max. NA's
#> 0.000 8.598 30.233 33.733 55.691 97.906 4241
raster::plot(dem_lisbon_raster)
# A vector dataset included in the package:
class(lisbon_road_segments)
#> [1] "sf" "tbl_df" "tbl" "data.frame"
plot(sf::st_geometry(lisbon_road_segments), add = TRUE)
```

Calculate the average gradient of each road segment as follows:

```
lisbon_road_segments$slope = slope_raster(lisbon_road_segments, e = dem_lisbon_raster)
summary(lisbon_road_segments$slope)
#> Min. 1st Qu. Median Mean 3rd Qu. Max.
#> 0.00000 0.01246 0.03534 0.05462 0.08251 0.27583
```

This created a new column, `slope`

that represents the average, distance weighted slope associated with each road segment. The units represent the percentage incline, that is the change in elevation divided by distance. The summary of the result tells us that the average gradient of slopes in the example data is just over 5%. This result is equivalent to that returned by ESRI’s `Slope_3d()`

in the 3D Analyst extension, with a correlation between the ArcMap implementation and our implementation of more than 0.95 on our test dataset (we find higher correlations on larger datasets):

```
cor(
lisbon_road_segments$slope, # slopes calculates by the slopes package
lisbon_road_segments$Avg_Slope # slopes calculated by ArcMap's 3D Analyst extension
)
#> [1] 0.9770436
```

We can now visualise the slopes calculated by the `slopes`

package as follows:

`# mapview::mapview(lisbon_road_segments["slope"], map.types = "Esri.WorldStreetMap")`

Imagine that we want to go from Santa Catarina to the East of the map to the Castelo de Sao Jorge to the West of the map:

`mapview::mapview(lisbon_route)`

We can convert the `lisbon_route`

object into a 3d linestring object as follows:

`lisbon_route_3d = slope_3d(lisbon_route, dem_lisbon_raster)`

We can now visualise the elevation profile of the route as follows:

`plot_slope(lisbon_route_3d)`

If you do not have a raster dataset representing elevations, you can automatically download them as follows.

```
lisbon_route_3d_auto = slope_3d(r = lisbon_route)
#> Preparing to download: 12 tiles at zoom = 15 from
#> https://api.mapbox.com/v4/mapbox.terrain-rgb/
plot_slope(lisbon_route_3d_auto)
```

For this benchmark we will download the following small (< 100 kB) `.tif`

file:

```
u = "https://github.com/ITSLeeds/slopes/releases/download/0.0.0/dem_lisbon.tif"
if(!file.exists("dem_lisbon.tif")) download.file(u, "dem_lisbon.tif")
```

A benchmark can reveal how many route gradients can be calculated per second:

```
e = dem_lisbon_raster
r = lisbon_road_segments
# et = terra::rast("dem_lisbon.tif")
et = terra::rast(u)
res = bench::mark(check = FALSE,
slope_raster = slope_raster(r, e),
slope_terra = slope_raster(r, et)
)
```

```
res
#> # A tibble: 2 x 6
#> expression min median `itr/sec` mem_alloc `gc/sec`
#> <bch:expr> <bch:tm> <bch:tm> <dbl> <bch:byt> <dbl>
#> 1 slope_raster 76.2ms 79.4ms 12.5 5.72MB 5.01
#> 2 slope_terra 608.8ms 608.8ms 1.64 2.21MB 0
```

That is approximately

routes per second using the `raster`

and `terra`

(the default if installed, using `RasterLayer`

and native `SpatRaster`

objects) packages to extract elevation estimates from the raster datasets, respectively.

The message: use the `terra`

package to read-in DEM data for slope extraction if speed is important.

To go faster, you can chose the `simple`

method to gain some speed at the expense of accuracy:

```
e = dem_lisbon_raster
r = lisbon_road_segments
res = bench::mark(check = FALSE,
bilinear1 = slope_raster(r, e),
bilinear2 = slope_raster(r, et),
simple1 = slope_raster(r, e, method = "simple"),
simple2 = slope_raster(r, et, method = "simple")
)
```

```
res
#> # A tibble: 4 x 6
#> expression min median `itr/sec` mem_alloc `gc/sec`
#> <bch:expr> <bch:tm> <bch:tm> <dbl> <bch:byt> <dbl>
#> 1 bilinear1 76.9ms 79ms 12.7 5.72MB 5.07
#> 2 bilinear2 491.4ms 491.4ms 2.03 2.16MB 2.03
#> 3 simple1 67.8ms 73.1ms 13.9 2.05MB 5.56
#> 4 simple2 555.5ms 555.5ms 1.80 2.17MB 0
```