Accessing Spatial Data from NLS OGC apis with geofi

Introduction

The geofi package provides tools to access spatial data from the Maastotietokanta (Topographic Database) and Geographic Names (Nimistö) datasets provided by the National Land Survey of Finland (NLS) via their OGC API. This vignette demonstrates how to use the package’s core functions to:

List available collections in the Maastotietokanta.
Download specific collections of spatial data.
Query geographic place names with flexible filtering options.

The package handles API authentication, pagination, spatial filtering, and coordinate reference system (CRS) transformations, making it easy to work with Finnish spatial data in R.

To use the package, you need an API key from the National Land Survey of Finland. Visit their website to obtain one. Once you have the key, set it in R using:

options(geofi_mml_api_key = "your_api_key_here")

Package Overview

The geofi package includes the following key functions:

ogc_get_maastotietokanta_collections(): Retrieves a list of available collections from the Maastotietokanta, including their titles and descriptions.
ogc_get_maastotietokanta(): Downloads a specific collection from the Maastotietokanta, with options for CRS, spatial filtering, and pagination.
ogc_get_nimisto(): Queries the Geographic Names dataset, allowing filtering by search string, bounding box, and other parameters.
fetch_ogc_api_mml(): An internal function that handles low-level API requests and pagination (not typically called directly by users).

All functions return spatial data as sf objects, compatible with the sf package for spatial analysis and visualization.

Step 1: Listing Available Collections

To explore the datasets available in the Maastotietokanta, use ogc_get_maastotietokanta_collections(). This function queries the OGC API and returns a data frame with collection IDs and descriptions.

collections <- ogc_get_maastotietokanta_collections()
head(collections)

This list helps you identify the collection parameter needed for ogc_get_maastotietokanta().

Step 2: Downloading a Maastotietokanta Collection

The ogc_get_maastotietokanta() function downloads a specific collection as an sf object. You can customize the output with parameters like:

collection: The name of the collection (e.g., “hautausmaa” for cemeteries).
crs: The coordinate reference system (EPSG:3067 or EPSG:4326).
limit: Maximum number of features per request (or NULL to fetch all).
bbox: A bounding box for spatial filtering (in EPSG:4326).
max_pages: Maximum number of pages to fetch when paginating.

Example: Downloading Cemeteries

Let’s download the “hautausmaa” collection in the default CRS (EPSG:3067):

cemeteries <- ogc_get_maastotietokanta(collection = "hautausmaa", crs = 4326)
cemeteries

Example: Spatial Filtering with a Bounding Box

To download cemeteries within a specific area (e.g., around Helsinki), use the bbox parameter. Coordinates must be in EPSG:4326 (WGS84).

cemeteries_helsinki <- ogc_get_maastotietokanta(
  collection = "hautausmaa",
  bbox = "24.5,60.1,25.5,60.5",
  crs = 4326
)

Visualize the results using ggplot2:

ggplot(cemeteries_helsinki) +
  geom_sf() +
  theme_minimal() +
  labs(title = "Cemeteries near Helsinki")

Example: Handling Large Collections

For large collections like “suo” (bogs/marshes), you may need to increase max_pages to fetch all features:

bogs <- ogc_get_maastotietokanta(
  collection = "suo",
  max_pages = 15
)

Note: Large collections may take significant time to download. If max_pages is reached, a warning will indicate that additional features may exist.

Step 3: Querying Geographic Names

The ogc_get_nimisto() function retrieves place names from the Geographic Names dataset. You can filter results by:

search_string: A case-insensitive search term (e.g., “kainu”).
bbox: A bounding box for spatial filtering.
limit: Maximum number of features to retrieve.
crs: Output CRS (EPSG:3067 or EPSG:4326).

Example: Searching for Place Names

Search for place names containing “kainu”:

kainu_places <- ogc_get_nimisto(search_string = "kainu")
print(kainu_places)

Visualize the results:

ggplot(kainu_places) +
  geom_sf() +
  geom_sf_text(aes(label = spelling), size = 3, check_overlap = TRUE) +
  theme_minimal() +
  labs(title = "Place Names Containing 'Kainu'")

Example: Combining Search and Spatial Filtering

Search for “kainu*” (with * wildcard) within a bounding box covering most part of Kainuu-region:

kainu_bbox <- ogc_get_nimisto(
  search_string = "kainu*",
  bbox = "27.515259,63.450509,30.531006,64.524823",
  crs = 4326
)

Advanced Features

Pagination

When limit = NULL in ogc_get_maastotietokanta() or ogc_get_nimisto(), the package automatically paginates through large datasets. The fetch_ogc_api_mml() function handles this internally, attempting to fetch all features in one request (limit=-1) or falling back to paginated requests if needed. Use max_pages to control the maximum number of pages fetched.

Error Handling

The package includes robust error handling:

Validates inputs (e.g., API key, CRS, bounding box format).
Retries failed requests up to 3 times for transient network issues.
Handles HTTP 429 (rate limit) errors by respecting the Retry-After header.
Provides informative error messages for API failures or invalid responses.

Custom Parameters

Function ogc_get_nimisto() support custom_params for advanced API queries. For example:

swimming_beaches_filtered_by_municipality_number <- ogc_get_nimisto(
  search_string = "*uimaranta*",
  custom_params = "municipality=091"
)
swimming_beaches_filtered_by_municipality_number

Consult the NLS OGC API documentation for valid parameters.

Best Practices

API Key Management: Store your API key securely using options(geofi_mml_api_key = "your_key") rather than hardcoding it in scripts.
Large Datasets: For collections like “suo”, test with a small limit or bbox to estimate runtime before fetching all features.
CRS Selection: Use EPSG:3067 (ETRS-TM35FIN) for Finnish data unless you need WGS84 (EPSG:4326) for compatibility with other systems.
Check Available Collections: Always use ogc_get_maastotietokanta_collections() to verify collection names before downloading.