Will direct visualisation of remote sensing data replace the topographic map?

Author: Pyry Kettunen

Kuva esittää korkeuskäyriä.

Contours are a good example of the condensed content of topographic maps that inherents from the historical development of the cartography and functions efficiently also in contemporary maps (figure: Pyry Kettunen).

The intense development of georeferenced photographs, laser scanning and satellite positioning has lifted the amount and accuracy of different remote sensing data so huge that knowledge about the geometries and attributes of ground surface, vegetation and constructions is about to be directly readable from these data alone. Satellite, aerial and panorama images as ortho-rectified, oblique or street images provide accurate and visually easily interpretable information about the composition of the surface of terrain.

Due to partial penetration inside vegetation and constructions, laser-born point clouds and digital elevation models allow for viewing the structure of terrain deeper than photographs. When these are still enriched with surface classification from multi-spectral laser scanning in near future, the whole data ensemble begins to seem perfect in conveying terrain information. The traditional topographic map is about to be at stake – does it not?

Direct visualisation supports accurate analysis

Above-listed terrain data are portrayals of physical terrain that are mechanically measured and carry well-defined technical qualities. The homogeneousness of these data makes them reliable as each point of terrain is known to be observed in the very same measure. The data ensembles thus create technically favourable preconditions for comparing different areas and their qualities. For the most part, the users of the data perform this comparison through direct visualisation. By direct visualisation I mean the depiction of the data as such, without transforming geometries or attributes, for instance, rendering a point cloud on a screen with point types classified by colour.

Direct visualisation makes the users see a certain measured reality through the data. The users recognise familiar geometric forms and learn to interpret the information conveyed by the data in accordance to their needs, for example, when interpreting classified point clouds. Since information by the data is semantically limited, the users' interpretational skills and knowledge on both the terrain and characteristics of the visualisation are central for successful reading. In addition, direct visualisations typically contain random errors or other undesired artifacts that cannot be automatically removed from the data. The users must be able to ignore these from their interpretation.

A topographic map disseminates semantically rich view on terrain

According to a general definition of a map, a topographic map is a two-dimensional geospatial image of ground surface downscaled with generalisation and abstracted by symbolic presentation. Loosely expressed, a topographic map is a large-scale generic map that depicts features of the physical reality that people can visually recognise on the ground level.

Representing landforms is a central characteristic of topographic maps, which is typically realised by contours, but topographic maps also contain information about other permanent features formed by nature or people. Topographic maps can be used for most diverse needs of geospatial perception. For instance, familiarising with a terrain or navigating in a terrain are significant purposes of use.

From the user's point of view, a topographic map is a dense geospatial representation that requires knowledge of map symbols. However, when under control, it provides a comprehensive tool for understanding environment and for planning action in the environment due to its versatile content and focused depiction of essential terrain features. A topographic map allows for direct reading of networks and fairways for all locomotion modalities, humanely significant information about buildings and terrain (for example, churches and nature reserves) and information about the structure of the nature, such as forest type or water flow. All this information has been captured from multiple processed sources with focus on reliability of the information. This comprehensiveness allows for possibilities to interpret the map image in relation to varying needs of geospatial perception that the users may have.

An ideal topographic map can be compared to an ideally neutral and reliable news publication that condenses news items from different sources to a uniform and full report of comprehensive essential aspects about the topic.

A central quality of a topographic map is its graphic representation developed through centuries. The founding driver of designing topographic maps in all times has been the optimal service towards the map reader. Thus, the graphics have always been refined to the utmost limits of available representation techniques, which makes the map an archetype in the design of human-technique interaction. Another similarly constitutive and developmentally comparable technical discipline has been architecture, for which spatial innovations and advances in construction techniques have shaped the discipline to how we see it today.

An unnecessary confrontation

The above factors may make the reader realise how different geospatial representations the directly visualised remote sensing data and the topographic maps are. Laying them into positions of confrontation as sketched in this article should not be necessary but, rather, harmful for the development of geospatial communication and pictorial representation about terrain. Direct visualisation supports excellently measurement and analysis whereas a topograpic map is at its best in conveying semantically rich perception of terrain and for considering the multiple aspects of terrain even simultaneously. Processing remote sensing and surveying data into topographic and other humanely informative maps seems to be highly necessary also in the future world of transcendent measuring techniques.

Reading:

  • Hoarau C., 2015. Représentations cartographiques intermédiaires. Comment covisualiser une carte et une orthophotographie pour naviguer entre abstraction et réalisme ?
  • Kettunen P., Irvankoski K., Krause C. M., Sarjakoski T. and L. T. Sarjakoski, 2012. Geospatial images in the acquisition of spatial knowledge for wayfinding. Journal of Spatial Information Science 5: 75–106.
  • Niemelä O., 2004. Maasto ja kartta. Kartanvalmistajan ja -käyttäjän käsikirja. Maanmittauslaitos.

D.Sc. (Tech.) Pyry Kettunen acts as a Senior Research Scientist in the Finnish Geospatial Research Institute (FGI) in the Department of Geoinformatics and Cartography. E-mail: pyry.kettunen@nls.fi.