How Web Maps Work
A Web map is not normally transferred as a single large image. The territory is generally divided into small portions, called tiles, which are requested by the software according to the displayed area and zoom level.
This system makes it possible to quickly display even very large geographic areas. When the user moves the map or changes the zoom level, OkMap requests the images or data from the service needed to build the new view.
Web maps can use different methods to provide these data. Some services distribute pre-generated tiles, while others dynamically generate the map image in response to the client request.
Tiled (XYZ)
The Tiled (XYZ) format uses a structure based on tiles arranged in a grid and divided into zoom levels. Each tile is identified by the coordinates of its position in the grid and the zoom level.
This approach is particularly efficient for interactive map display, because OkMap only needs to request the tiles required for the currently displayed area. It is the method used by many Web mapping services and is one of the most common methods for distributing online maps.
Tiled services are also often referred to as XYZ. The tile URL structure generally contains the zoom, X and Y values needed to identify each portion of the map.
A Tiled (XYZ) service normally provides a single map layer Each request therefore returns the tiles for that specific map. To use information from different services, you can load multiple Web maps and overlay them.
WMS (Web Map Service)
WMS (Web Map Service) is a mapping service standardized by the Open Geospatial Consortium (OGC) that generates a map image in response to a request.
The request can specify the geographic area, the coordinate reference system, the image dimensions and the layers to display. The server processes this information and returns the requested map image, normally in a raster format such as PNG or JPEG.
Unlike services based on predefined tiles, WMS therefore generates the image according to the request. This makes the service particularly flexible for displaying cartographic and thematic data.
A WMS service can provide multiple layers, each corresponding to a different type of cartographic information. In OkMap the user can choose which layers to display and combine them to obtain the desired representation.
WMTS (Web Map Tile Service)
WMTS (Web Map Tile Service) is a service standardized by the Open Geospatial Consortium (OGC) that distributes maps through predefined tiles organized according to zoom levels and a standardized spatial structure.
Compared with WMS, the server normally does not have to generate each image when it is requested. The tiles are already prepared and can therefore be transferred quickly to the client.
WMTS is particularly suitable for interactive display of large amounts of cartography, while WMS offers greater flexibility in generating the requested images.
A WMTS service can also provide multiple layers, for example base maps, imagery or thematic data. In OkMap the user can select the desired layers and display them simultaneously.
PMTiles Raster and PMTiles Vector
PMTiles is a method for distributing Web maps in which tiles are organized within an archive accessible over the Internet. OkMap supports both PMTiles raster and PMTiles vector.
A PMTiles raster archive contains raster tiles, i.e. map images. The content is therefore already represented as images, similarly to what happens with a normal raster map divided into tiles.
A PMTiles vector archive instead contains vector data organized in tiles. In this case the geographic content is not a simple image, but geometries and attributes that can be rendered by the software during display.
The difference between the two methods is therefore similar to that between a raster map and a vector map: in the first case the software displays predefined images, while in the second case it displays vector geographic data.
Comparison of the Main Types of Web Maps
The different types of Web maps have different characteristics and there is no universally best format. The choice depends on the type of data available and the requirements of the service.
| Type | How the data are provided | Layer | Main characteristic |
|---|---|---|---|
| Tiled (XYZ) | Raster tiles organized by zoom and position | Normally a single layer | Fast and widespread Web display |
| WMS | Images generated by the server | One or more selectable layers | High flexibility in map requests |
| WMTS | Predefined tiles | One or more selectable layers | Fast and standardized distribution |
| PMTiles raster | Raster tiles contained in a PMTiles archive | Depends on the content | Distribution of tiled raster data through a single archive |
| PMTiles vector | Vector tiles contained in a PMTiles archive | Depends on the content | Distribution of tiled vector geographic data |
Web Maps and Coordinate Systems
Web maps must be associated with a coordinate reference system consistent with the geographic data being displayed. Many Web mapping services use the Web Mercator, which is particularly suitable for interactive display of maps through tiles.
OkMap automatically handles the conversions required to overlay Web maps with other supported cartographic data, such as local raster maps, GPX tracks and waypoints. Correct definition of the coordinate reference system remains important to achieve accurate alignment of the data.
Using Web Maps with OkMap
In OkMap Desktop you can load a Web map directly from an Internet server. The software provides a list of predefined services and allows you to configure new custom services by specifying their access parameters.
Once a Web map has been loaded, it can be displayed together with the other cartographic data in the project. Web maps can be used as a base for planning routes, displaying GPX tracks and waypoints, and analyzing the territory.
OkMap also allows you to use multiple maps simultaneously, organizing them as overlaid layers. It is therefore possible to combine a Web map with other Web maps or with a raster map and adjust its transparency.
Online Web Maps and Offline Maps
A Web map normally requires an Internet connection to retrieve data from the service. This is particularly convenient when you want to use up-to-date maps without using disk space for large amounts of data.
To use the map even without a connection, OkMap allows you to download a portion of the Web map and save it locally. You can therefore prepare in advance the area of interest, choose the zoom level and create a map that can later be used offline.
Downloading a Web map must always comply with the terms of use and any limitations established by the service provider. Not all services allow bulk downloading or offline use of their data.
Web Maps and GPS Data
Web maps can be used as a map base during GPS data planning and analysis. In OkMap it is possible to display track, routes and waypoint, using them to plan routes or analyze completed routes.
It is also possible to combine different Web maps to obtain complementary information. For example, a topographic map can be used as a base and another thematic map can be overlaid with an appropriate transparency level.