Mapping constitutes an integral component of the process of managing land resources, and mapped information is the common product of analysis of remotely sensed data. Natural features and manufactured infrastructures, such as transportation networks, urban areas, and administrative boundaries can be presented spatially with respect to referenced co-ordinate systems, which may then be combined with thematic information. Baseline, thematic, and topographic maps are essential for planning, evaluating, and monitoring, for military or civilian reconnaissance, or land use management, particularly if digitally integrated into a geographic information system as an information base. Integrating elevation information is crucial to many applications and is often the key to the potential success of present day mapping programs.
Canada has been, and continues to be a world leader in mapping technology. Canada's immense land area with a rich resource potential, coupled with a small population base has necessitated the development of thorough and efficient mechanisms of investigating and recording land information. Traditionally, this information was obtained through surveying and photogrammetric techniques, which have been costly and time consuming, particularly for periodic revision of outdated information. Recent advances in computer technology (speed, data handling and storage capability) and a growing demand for digital databases and computer based mapping production capabilities have encouraged the use of remotely sensed information as a data source for cartographic applications.
There is a growing demand for the utilization of remote sensing data in map production, since the following benefits may be provided: stereo coverage, frequent revisits, timely delivery, wide area coverage, low labour intensity, virtually global coverage, and storage in digital format to facilitate subsequent updating and compatibility with current GIS technology.
End users of base maps and mapping products include resource companies (forestry, mining, oil), support and service industries (engineering), utility and infrastructure development agencies (pipelines, telecommunications, transportation, power), government mapping agencies, and the military. This diversification from traditionally military users to commercial applications has resulted in a greater demand for a wider range of mapping products, with emphasis placed upon the benefits of improved information content and scale, and accuracy versus data costs.
Canadian companies offering mapping services are likely to be looking abroad, as the greatest commercial potential exists within the international community. Developing countries are currently initiating mapping programs to cover large unsurveyed areas to increase their topographic and planimetric knowledge base. The derived information will be used to support territorial sovereignty issues, assess and monitor resource potential and exploitation, and encourage economic opportunity. Radar data will be relied on in tropical areas for remote sensing mapping solutions.
Mapping applications of remote sensing include the following:
Planimetry consists of the identification and geolocation of basic land cover (e.g. forest, marsh), drainage, and anthropogenic features (e.g. urban infrastructure, transportation networks) in the x, y plane. Planimetric information is generally required for large-scale applications - urban mapping, facilities management, military reconnaissance, and general landscape information.
Why remote sensing?
Land surveying techniques accompanied by the use of a GPS can be used to meet high accuracy requirements, but limitations include cost effectiveness, and difficulties in attempting to map large, or remote areas. Remote sensing provides a means of identifying and presenting planimetric data in convenient media and efficient manner. Imagery is available in varying scales to meet the requirements of many different users. Defence applications typify the scope of planimetry applications - extracting transportation route information, building and facilities locations, urban infrastructure, and general land cover.
Very high resolution is usually a requirement for accurate planimetric mapping. Concerns of the mapping community with regard to use of satellite data are spatial accuracy and the level of detail of extractable information content. The concern for information content focusses not only on interpretability of features, but on the ability to determine the correct spatial location of a feature. An example of the latter would be the difficulty associated with defining the centre of a river or precise location of a powerline or pipeline right-of-way in vector format, when interpreting from a relatively coarse raster base. Spatial resolution is a critical element in this case.
The turnaround time of one or two weeks will generally meet the requirements for this type of mapping, although defence requirements may be more stringent.
Canada vs. International
For general Canadian applications, the ability to provide planimetric information is best addressed by current VIR sensors, and for large scale mapping- aerial photography. The importance of adequate resolution and information content outweigh the need for near real time products. Presently, TM and SPOT data provide optimal information for extracting planimetric information for regional applications. Air photos, and particularly orthophotos when available, are preferred for smaller, well defined areas.
For cloud covered areas, radar is the obvious choice for providing planimetric data. The detectability of linear features improves when they are oriented perpendicular to the radar look direction. This can be controlled with airborne sensors, by planning the flightlines appropriately. Another issue is that a balance between resolution and speckle has to be reached. Although single look data provides the finest resolution, speckle can be a hindrance to interpretation, and invites multilook processing.
The availability of digital elevation models (DEMs) is critical for performing geometric and radiometric corrections for terrain on remotely sensed imagery, and allows the generation of contour lines and terrain models, thus providing another source of information for analysis.
Present mapping programs are rarely implemented with only planimetric considerations. The demand for digital elevation models is growing with increasing use of GIS and with increasing evidence of improvement in information extracted using elevation data (for example, in discriminating wetlands, flood mapping, and forest management). The incorporation of elevation and terrain data is crucial to many applications, particularly if radar data is being used, to compensate for foreshortening and layover effects, and slope induced radiometric effects. Elevation data is used in the production of popular topographic maps.
Elevation data, integrated with imagery is also used for generating perspective views, useful for tourism, route planning, to optimize views for developments, to lessen visibility of forest clearcuts from major transportation routes, and even golf course planning and development. Elevation models are integrated into the programming of cruise missiles, to guide them over the terrain.
Resource management, telecommunications planning, and military mapping are some of the applications associated with DEMs.
Why remote sensing?
There are a number of ways to generate elevation models. One is to create point data sets by collecting elevation data from altimeter or Global Positioning System (GPS) data, and then interpolating between the points. This is extremely time and effort consuming. Traditional surveying is also very time consuming and limits the timeliness of regional scale mapping.
Generating DEMs from remotely sensed data can be cost effective and efficient. A variety of sensors and methodologies to generate such models are available and proven for mapping applications. Two primary methods if generating elevation data are 1. Stereogrammetry techniques using airphotos (photogrammetry), VIR imagery, or radar data (radargrammetry), and 2. Radar interferometry.
Stereogrammetry involves the extraction of elevation information from stereo overlapping images, typically airphotos, SPOT imagery, or radar. To give an example, stereo pairs of airborne SAR data are used to find point elevations, using the concept of parallax. Contours (lines of equal elevation) can be traced along the images by operators constantly viewing the images in stereo.
The potential of radar interferometric techniques to measure terrain height, and to detect and measure minute changes in elevation and horizontal base, is becoming quickly recognized.
Interferometry involves the gathering of precise elevation data using successive passes (or dual antenna reception) of spaceborne or airborne SAR. Subsequent images from nearly the same track are acquired and instead of examining the amplitude images, the phase information of the returned signals is compared. The phase images are coregistered, and the differences in phase value for each pixel is measured, and displayed as an interferogram. A computation of phase "unwrapping" or phase integration, and geometric rectification are performed to determine altitude values. High accuracies have been achieved in demonstrations using both airborne (in the order of a few centimetres) and spaceborne data (in the order of 10m).
Primary applications of interferometry include high quality DEM generation, monitoring of surface deformations (measurement of land subsidence due to natural processes, gas removal, or groundwater extraction; volcanic inflation prior to eruption; relative earth movements caused by earthquakes), and hazard assessment and monitoring of natural landscape features and fabricated structures, such as dams. This type of data would be useful for insurance companies who could better measure damage due to natural disasters, and for hydrology-specialty companies and researchers interested in routine monitoring of ice jams for bridge safety, and changes in mass balance of glaciers or volcano growth prior to an eruption.
From elevation models, contour lines can be generated for topographic maps, slope and aspect models can be created for integration into (land cover) thematic classification datasets or used as a sole data source, or the model itself can be used to orthorectify remote sensing imagery and generate perspective views.
The basic data requirement for both stereogrammetric and interferometric techniques is that the target site has been imaged two times, with the sensor imaging positions separated to give two different viewing angles.
In virtually all DEM and topographic map generation applications, cartographic accuracy is the important limiting factor. Turnaround time is not critical and repeat frequency is dependent on whether the application involves change detection, and what the temporal scope of the study is.
Canada vs. International
Aerial photography is the primary data source for DEM generation in Canada for national topographic mapping. For other applications of DEMs, there are additional satellite sources such as SPOT, with its pointable sensors and 10m panchromatic spatial resolution, producing adequate height information at scales smaller than 1:50,000.
The height accuracy requirement for 1:50,000 mapping in Canada is between 5 and 20 m. In developing countries it is typically 20 m. The original elevation information used in the Canadian National Topographic Series Maps was provided from photogrammetric techniques.
In foreign markets, airborne radar mapping is most suited for approximately 1:50,000 scale topographic mapping. Spaceborne radar systems will be able to provide data for the generation of coarser DEMs through radargrammetry, in areas of cloud cover and with less stringent accuracy requirements. Stereo data in most modes of operation will be available because of the flexible incidence angles, allowing most areas to be captured during subsequent passes. Interferometry from airborne and spaceborne systems should meet many mapping requirements.
When you look at a stereo pair of images you perceive a virtual 3D model of the terrain or object that was imaged. Through this 3D virtual terrain model (VTM?), it is possible to extract cartographic information without using a DEM!
There is a growing demand for digital databases of topographic and thematic information to facilitate data integration and efficient updating of other spatially oriented data. Topographic maps consist of elevation contours and planimetric detail of varied scale, and serve as general base information for civilian and military use.
Baseline thematic mapping (BTM) is a digital integration of satellite imagery, land use, land cover, and topographic data to produce an "image map" with contour lines and vector planimetry information. This new concept of thematic mapping was developed to take advantage of improvements in digital processing and integration of spatial information, increased compatibility of multisource data sets, the wide use of geographic information systems to synthesize information and execute analyses customized for the user, and increased ability to present the data in cartographic form.
The data for baseline thematic maps are compiled from topographic, land cover, and infrastructure databases. Appropriate thematic information is superimposed on a base map, providing specific information for specific end users, such as resource managers. Various combinations of thematic information may be displayed to optimize the map information for application specific purposes, whether for land use allocation, utility site selection and route planning, watershed management, or natural resource management and operations.
Why remote sensing?
As a base map, imagery provides ancillary information to the extracted planimetric or thematic detail. Sensitivity to surface expression makes radar a useful tool for creating base maps and providing reconnaissance abilities for hydrocarbon and mineralogical companies involved in exploration activities. This is particularly true in remote northern regions, where vegetation cover does not mask the microtopography and generally, information may be sparse. Multispectral imagery is excellent for providing ancillary land cover information, such as forest cover. Supplementing the optical data with the topographic relief and textural nuance inherent in radar imagery can create an extremely useful image composite product for interpretation.
The prime data requirement is for high information content and a balance between resolution and data handling ability. There is a moderate turnaround requirement for this application; processed data should be available less than a year after imagery acquisition.
Canada vs. International
VIR imagery is excellent as a base map for planimetry detail on a varied landscape, providing information on forest, agriculture cover and gross geomorphology of the land. SAR is also good for providing surficial topographic expression.
Case study (example) BTM's in BC
(Baseline Thematic Mapping in British Columbia)
Baseline thematic mapping involves the compilation of varied data sources, ranging from satellite imagery to detailed forest stand information to planimetric data from the 1:250,000 National Topographic database. Base map sheets overlain by various combinations of thematic data are produced with an aim toward resource management applications. British Columbia's Ministry of Environment, Lands, and Parks routinely produces BTMs. The most recent Landsat TM data available is used as a source for classifications of ground cover and interpretation of land use. DEMs are also integrated into the satellite data to provide 3 dimensional perspective views. Although B.C. is quite advanced in this application, other Canadian provinces have contemplated or are doing similar work, as are private consultants in conjunction with forestry companies.
Baseline thematic mapping incorporates not only interpretations of ground cover data and land use, but topographic information such as elevation contours and planimetry to provide an optimal tool for resource management. This information may be portrayed in traditional map format, or as an image-map, which is an excellent means of presenting spatial data to resource managers and many other users.
A 'close' relative of 3D terrain mapping is 'close range photogrammetry'. Using very similar techniques but at very close range, this method is used for 'mapping' an object like a building, sculpture or a human face in three dimensions in order to have a precise record of its shape.