THE USE OF SATELLITE AND AERIAL IMAGERY, AND GPS FOR DELINEATION OF EA BOUNDARIES Roberto Bianchini
Summary 1. Objectives of delineation of EA boundaries 2. Criteria for the delineation of EA boundaries 3. Main critical criterion to delineate EA boundaries 4. Principal type of maps for census operations 5. The satellite remote sensing 6. The aerial photography 7. The Global Positioning system (GPS) 8. Some conclusions
1. Objectives of delineation of EA boundaries To support the census coverage To support fieldwork operations To contribute to the statistical analysis at small area level To support sampling for statistical surveys
2. Criteria for the delineation of EA boundaries EAs has: To cover the entire country and do not have to overlap Be consistent with the administrative division and other relevant geographic units To have boundaries easily identifiable on the ground To be of approximately equal population size in order to be covered by an enumerator during data collection (Differences in the population size may occur for urban and rural areas) To be consistent (to a larger extent) with EA boundaries used in the previous census To be large enough to guarantee data privacy
3. Main critical criterion to delineate EA boundaries The main critical criterion is to estimate the approximately equal population size of EAs The three methods are: - Acquisition of administrative records (or by other sources of information) - Fieldwork activities (dwellings count) - Through remotely sensed data (satellite imagery and aerial photographs), and GPS receivers
4. Principal type of maps for census operations Small scale maps Topographic maps City maps Sketch maps High-resolution satellite images and aerial photos
4. Small scale map
4. Topographic map
4. City map
4. Sketch map
4. Manual EA delineation
4. High-resolution satellite imagery
4. Aerial photograph
5. Satellite remote sensing The spatial resolution of a satellite image is measured by the size of a pixel on the ground. The size for commercial satellites varies from the sub-one-meter of the most popular high-resolution systems such as Quickbird and Ikonos Indian Remote Sensing (IRS), SPOT s panchromatic sensor and Landsat multi-spectral imagery are also considered high resolution systems which allow mapping at cartographic scales of 1:25,000 to 1:50,000 or smaller In September 2008, the remote sensing company GeoEye launched a sensor with a pixel size of 0.41 meters panchromatic and 1.65-meter multispectral
5. Satellite remote sensing (cont.) Only some remotely sensed data products serve census work well, namely high spatial resolution data such as Geoeye (0.41), Quickbird (0.82m), Ikonos (1m), IRS (5.8m pan), Orbimage 3 and 4 (1m), and SPOT 5 (2.5m). Ikonos was launched in 1999 and Quickbird was launched in 2001. For most census applications, 5m or better spatial resolution is needed to identify housing units. Landsat may be considered in some cases only in rural areas On line remotely sensed data such as Google Earth, GlobeXplorer, ArcGIS Explorer, and Microsoft Virtual Earth can be used as a first test, but the image quality may not be suitable for detailed EA mapping
5. Satellite remote sensing (cont.) High resolution satellite images show a level of geographic detail that is almost comparable to digital orthophoto maps created from air photos. Cloud free high-resolution images may allow counts of single housing units Aerial photography is often done on ad hoc basis, and may be more suitable for detailed surveying and mapping projects. New digital aerial photography may offer superior image quality over even high-resolution satellite imagery
5. Satellite remote sensing (cont.) Main advantages and disadvantages of high-resolution remotely sensed data: Advantages: - High spatial resolution images offer the ability to cover large areas at a level of detail sufficient for EA delineation, provided population estimates exist for the areas delineated - Imagery can permit mapping of inaccessible areas - Usually there are no restrictions for their acquisition Disadvantages: - Cloud may cover restricts image interpretation - Positional accuracy may be low without accurate orthorectification process
6. Aerial photography Aerial photography is obtained using specialized cameras on-board low flying planes. The camera captures the image on photographic film, or digitally Traditionally, the end products of an aerial photography project are printed photos of an area on the ground. The photogrammetrist can combine these photos to produce a seamless mosaic covering very large part of a territory Nowadays, the most common approach is to convert the photo negative directly into a film transparency which is scanned using a very high resolution scanning device. The result is a digital image that can be displayed and processed further on a computer
6. Aerial photography (cont.) Additional processing is required to produce orthophoto maps which combine the geometrical accuracy of a topographic map with the large detail of a photograph The construction of digital orthophotos requires considerable expertise in photogrammetric methods, which is not usually present in a census organization The census organization therefore needs to establish a collaborative agreement with the National Mapping Agency
6. Aerial photography (cont.) Orthophoto maps may also support dwelling unit counts and population estimation, if is available a reliable estimate of the average number of persons per household In a rural setting, the number of dwelling units can be determined fairly easily, while in urban settings, the number of dwellings may be more difficult to determine Since the orthophotos are correctly georeferenced, the resulting enumeration areas will also be registered in a proper map projection with known parameters
6. Aerial photography (cont.) Main advantages and disadvantages of high-resolution remotely sensed data: Advantages: - Air photos provide a large amount of detail and can be interpreted visually - They usually cover the entire country Disadvantages: - Aerial photo processing requires expensive equipment and specialized expertise - Their production is in general more expensive in comparison with high-resolution satellite imageries and requires Governmental approval
Aerial photograph (Resolution 25 cm) Ikonos satellite imagery (Resolution 1 m)
7. Global Positioning System (GPS) Determine current position based on signals sent by a number of satellites GPS receivers collect the signals transmitted from more than 24 satellites 21 active satellites and three spares. The system is called NAVSTAR, and is maintained by the U.S. Department of Defense In each area of the world, five to eight GPS satellites are within the field of view of a user on the earth s surface
7. Global Positioning System (cont.)
7. Global Positioning System (cont.) GPS uses satellite signals to obtain latitude/longitude coordinate
7. Global Positioning System (cont.) The accuracy of inexpensive GPS receivers is normally around 10-20 meters for civilian applications. Altitude information is less reliable than latitude and longitude To obtain more accurate positions GPS readings have to be longer To improve accuracy, differential GPS (DGPS) system (and real time system) use correction information transmitted from a permanent base station with precisely known coordinates to correct the satellite signals (1-2 meters with non-geodetic receivers)
7. Global Positioning System (cont.) Other global satellite navigation systems are: - GLONASS, operated by the Ministry of Defense of the Russian Federation - Galileo, the European system which will probably starts operations in 2013
7. Global Positioning System (cont.) GPS in mapping activities: - support for the preparation of enumerator maps for census activities - Coordinates can be downloaded or entered manually into a GIS or other digital mapping system, where they can be combined with other georeferenced information - support for the delineation of of the administrative boundaries - locating housing units - creation of the street network
7. Global Positioning System (cont.) Use of GPS for georeferencing and orthorectification of remotely sensed data Example of Ikonos satellite image with GCPs for orthorectification
6. Global Positioning System (cont.) Advantages: - Fairly inexpensive, easy-to-use - Sufficient accuracy for many census mapping applications high accuracy achievable with differential correction - Collected data can be read directly into GIS databases making intermediate data entry or data Disadvantages: - Signal may be obstructed in dense urban or wooded areas - Standard GPS accuracy may be insufficient in urban areas making differential techniques necessary
8. Some conclusions Interuniversity Research Center for Sustainable Development High-resolution satellite imagery and aerial photographs may be used generally to estimate small area population size and for delineation of EAs in rural environment (where buildings are in general scattered, and with one or two storey) In urban areas, fieldwork activities for dwellings count seems not always possible to be avoided for a reliable delineation of EAs In this respect, the use of GPS may support considerably fieldwork activities, if the base map (Satellite imagery, aerial photo or other digital cartography) is accurately georeferenced and orthorectified In general, the most suitable strategy seems to be the change-detection approach, by investigating in the field only areas with important changes, detected in office trough the analysis of remote sensed data
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