RDG Update: The Roman Empire Vector Map Project
We, Johan Åhlfeldt, Ida Storm and Petr Pridal, are happy to introduce our Pelagios grant sponsored project and want to start by expressing our thanks for the opportunity. This project aims at creating a tiled vector map to replace the current raster tiles of the Digital Atlas of the Roman Empire (DARE), https://dare.ht.lu.se. The projects builds on Open Source technology, https://openmaptiles.org, developed by Klokan Technologies GmbH and data from the DARE gazetteer and geographical information system as well as other openly available data. The resource created by this project will be licensed with a Creative commons (CC-BY) license, data and programming code will be made available on Github.
In 2012 Johan Åhlfeldt developed a tiled map of the Roman Empire in collaboration with the Pelagios project. A tiled map is a catalogue of pre-rendered raster images (png format) in size of 256×256 pixels. The tiles are referenced by zoom level and by x and y coordinate positions. At zoom level 0, the entire world is represented by one tile, at zoom level 1, four tiles and at subsequent zoom levels, the number of tiles are increasing exponentially. In the following year, we created the Digital Atlas of the Roman Empire (DARE), a geographical information system (GIS) including a gazetteer of ancient place names and a digital map with a maximum zoom level of 11, corresponding to a scale of 1:250 000. The number of ancient places in the gazetteer and on the digital map has increased since the start and is now around 27,600. Both the gazetteer and the tiled map are published with an Open license and all parts of the GIS are reusable. There are almost 20 research projects around the world that are using the tiled map and the gazetteer is part of the tools Recogito and Peripleo created by the Pelagios project. The DARE gazetteer has grown particularly strong when it comes to location accuracy and connecting other gazetteers and cultural heritage resources of the Ancient Roman and Greek world using a linked data approach. The DARE information system also integrates national topographical atlases and satellite images.
Vector tiles technology
The current raster based technology sets limits to the area covered by the map and number of zoom levels that can be served. Furthermore, it takes a lot of resources to keep the map updated as the number of places increases and the maps need to be re-rendered. In recent years there has been a shift of technology towards using a more flexible and adjustable solution building on vector data. One such solution is the Open map tiles technology developed by Klokan Technologies, https://openmaptiles.org. Instead of creating a set of static images (tiles) for every zoom level and x- and y-position, we can use the same vector data to render the tiles on the fly.
The project will be carried out by a project team at the Humanities lab, Lund University, Sweden, by Johan Åhlfeldt, Ida Storm, and by Petr Pridal, at Klokan Technologies, Switzerland. The project team in Lund will provide data currently used to produce the raster map a well as their expertise in designing historical information systems. Klokan technology will provide necessary technical expertise and prepared datasets from OpenStreetMap and Shuttle Radar Topography Mission (SRTM) by NASA. There is no fixed limit to the number of zoom levels as the data from which the tiles are rendered will be the same. As the user zooms in on the map, more details will be displayed.
The background layer of the map will be based on OpenStreetMap data for coastlines, rivers, lakes, and land cover. This data is already used in the current raster map of DARE. SRTM based data are hillshades, color-reliefs and contour lines. The map will have world coverage and the resolution of the SRTM based data will increase from 90 meter to 30 meter with the new version of the SRTM dataset.
Figure 1: The Klokantech Terrain style is based on OSM and SRTM data and will be used as the basis of the Roman Vector Map.
The look and feel of the current base map will be preserved and we will utilize the data that we already compiled for DARE, including places, roads, aqueducts and defense systems. The new vector map and the possibility to display higher zoom levels will benefit from the high level of location accuracy of the DARE gazetteer since one third of the places in DARE has an accuracy of 20 meter. It will even be possible to display ancient buildings and structures inside places at the higher zoom levels. Buildings have never been displayed on the current DARE raster map because of the relatively low zoom levels, but is already prepared for that since the gazetteer maintains links to OSM geometries for archaeological sites and structures. These links can be used for the selection of geometries relevant to the project.
Figure 2: OSM data tagged as “archaeological site” in Pompeii
The real novelty of the technology shift into vector tiles is that archaeological data in OpenStreetMap and other more scholarly resources can be utilized to render structures such as city walls, temples, amphitheaters etc. Some of these high quality OSM data resources will be included in the updated map. However, the main problem of a more general use of OSM-data is poor metadata to describe the geometries: there are seldom tags regarding the specific time-period of ancient structures or type of structure, which means there is no easy way to automatically subset only the ones belonging to the classical period.
The general trend in the Humanities is towards Open Data and there are a few datasets available that depicts archaeological structures of the Classical period and that we can use as examples, for instance, from the Archaeology Data Services (ADS), http://archaeologydataservice.ac.uk. We expect much more of this kind of data in the future, and with the shift to vector tiles we will be prepared to use them when they become available.
Impact and benefits for the Pelagios community
The many advantages of vector tiles are listed below.
- Custom styling which allows for easy change of look and feel to best fit different projects. This is achieved by separation of data and style
- Printing possibilities, export in 300+ DPI and Retina/HiDPI devices
- Fast to load, small in size, suitable to use on mobile devices, even offline
- Easy combination with other maps and custom data (in GeoJSON format)
- Mashups with standard tools such as OpenLayers and Leaflet
- Desktop GIS tools compatible – QGIS, ArcGIS
- Attractive visualization with animation and storytelling possibilities
- World coverage and possibility to display data from other gazetteers, e.g. China GIS at Harvard
- Easy to update vector tiles when new or improved data gets available, e.g. new or updated ancient places from the DARE gazetteer, archaeological sites in OpenStreetMap or scholarly data from other sources, e.g. Archaeology Data Services (AGS)
- Multiple languages in place name labels, dynamically switchable e.g. from Latin to Ancient Greek
From the list above, it is obvious that vector tiles is a small revolution compared to static raster tiles. From a technical point of view, this is already established technology and many of the problems that we currently face with raster tiles are simply not there anymore. Beside the clear advantages from the perspective of map producers (easy to update, smaller size, etc.), there are new features that could be left to the users of the map to decide. Many times, we have been asked if it’s possible to make a tiled map for a specific time-period and with Greek place names instead of Latin. With the shift to vector tiles it is possible to do both and the user is fully in charge to make, and re-make her choices: Selections can be based on any attribute in the place name data file (in GeoJSON format) including place type, time-period, language of place name etc.
With technology that supports these options, the only problem left to solve is to provide high quality data to make use of all the visualization possibilities. For instance, currently DARE only has 1400 Ancient Greek place names. A closer collaboration between gazetteers would help and Trismegistos Geo and ToposText are good examples of gazetteers that have put a lot of effort in compiling Ancient Greek place names. During this project, we will investigate the possibilities of a closer collaboration for the immediate purpose of bringing more Greek place names to the vector tiles.
The current raster map has played a big role in promoting the Pelagios collaboration and given it a visual expression. With the shift to vector tiles the scope and usability of the map will increase even more. Individuals and projects can customize the map with open source technology and be given this opportunity to a kick-start of the new technology.
Humanities Lab, Joint Faculty of Humanities and Theology, Lund University, Sweden