UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Integrating Fieldwork Using GPS and Remotely-Sensed Data

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Overview  Why using GPS and R/S for enumeration area delineation  Integrating Fieldwork Using GPS  Using satellite imagery to field-verify EA maps produced at census headquarters  Application of aerial photos for census mapping  Conclusion

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Why Using GPS & Remotely-Sensed Data  The main reason to do this is to field-validate the EA boundaries that were created in the NSO’s GIS lab from the prior census’s maps.  Or, when accurate maps are not available, it is done as a basis for EA delineation in the census main office, before conducting fieldwork for completion and validation.

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round The Approach  Through integration of satellite imagery, analysts and census planners can identify areas that require additional fieldwork, for instance to account for new growth in areas surrounding cities.  For planning and logistical purposes, it makes sense to identify these priority areas ahead of time to locate areas of rapid change since the last census and focus on them.  This is what is meant by a “change-detection” approach, and it can be most effectively done using a synthesis of field, lab, and remotely sensed data and largely involves GPS and Remotely Sensed data

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Integrating Fieldwork Using GPS  GPS technology offers many applications in mapping activities, including the preparation and correction of enumerator maps for census activities.  With DGPS, geographical positions of enumeration area (EA) boundaries can be corrected and the location of point features such as service facilities or village centers can be obtained in a cost-effective way.  Coordinates can be downloaded or entered manually into a GIS or other digital mapping system, where they can be combined with other georeferenced information.

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Integrating Fieldwork Using GPS  Some specific GPS-related mapping tasks EA boundary delineation  A preferred approach is to digitize EA boundaries from the previous census and use a ground-based GPS approach only where needed, particularly when boundary changes such as the creation of new districts or land annexations have occurred. Administrative boundary delineation  NSOs should weigh carefully the potential benefit of conducting detailed administrative boundaries at the time of the census against the cost in time and labor.

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Integrating Fieldwork Using GPS  Housing unit location – some countries have gone so far as to record a latitude-longitude for every housing unit in the country, sometimes even photographing each dwelling.  Collective living quarter locations – locating collective living quarters (communal or institutional housing) with GPS units may be less taxing since they are fewer in number than measuring all housing units in the country.  Other relevant features (including roads) – Features such as roads can be useful for delineating enumeration areas or for providing navigational information.

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Integrating Fieldwork Using GPS  The advantages of GPS include:  Fairly inexpensive, easy-to-use component of field data collection. Modern units require very little training for proper 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 conversion steps unnecessary;  Worldwide availability; and  New GPS systems will be coming on line in the next 5 years.

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Integrating Fieldwork Using GPS  The disadvantages are: Signal may be obstructed in dense urban or wooded areas (multi- path error); Standard GPS accuracy may be insufficient in urban areas and for capturing linear features making differential techniques necessary; DGPS is more expensive and may not be available in many remote places. DGPS requires more time in field data collection and more complex post-processing to obtain more accurate information; A very large number of GPS units may be required for only a short period of data collection, making widespread implementation of GPS potentially very expensive; and The more complex GPS unit measured, the more training is required.

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Using imagery to field-verify EA maps  since the advent of satellite imagery with high (1m or better) spatial resolution, remote sensing R/S has revolutionized mapping  Satellite imagery if used pragmatically can save countless person-hours by allowing the NSO to focus attention on critical areas  We advocate an approach that performs triage on the surface area of a country, partitioning it into areas needing more and less attention

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Using imagery to field-verify EA maps  With the provisional EA boundaries superimposed on R/S imagery population settlements can be quickly located and priority areas identified. EA boundaries delineated atop a panchromatic satellite image

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Using imagery to field-verify EA maps  Resolution of remote sensing data Only a relatively narrow range of R/S products serve census work well, namely high spatial resolution data such as Quickbird (0.82m), Ikonos (1m), IRS (5.8 m pan), Orbimage 3 and 4 (1m), and SPOT 5 (2.5m). Ikonos was launched in 1999 and Quickbird was launched in For most census applications, 5m or better spatial resolution is needed to identify housing units and the spread of population settlements, with multi-spectral imagery less absolutely necessary.

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Using imagery to field-verify EA maps Illustration of pixel size in aerial photographs and satellite images

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Very high spatial resolution civilian satellite remote sensing products ProductCompanyLaunchModePixel Size at nadir Height (km) QuickbirdDigital Globe2001Pan/4ms0.61/ Ikonos 2GeoEye1999Pan/4ms0.82/ OrbView 3OrbImage2003Pan/4ms1.0/ Spot 5SpotImage2002Pan/4ms5(2.5)/10830 Cartosat- 1 NASDA, Japan 2004Pan Cartosat- 2 NASDA, Japan 2004/5Pan1630

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Using imagery to field-verify EA maps  The advantages of satellite remote sensing data include: Up-to-date coverage of very large areas at relatively low cost with lower spatial resolution images; High spatial resolution images offer the ability to cover 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; Imagery can serve as an independent check on field verification; Update of topographic maps in rural areas is possible; e.g., identification of new settlements or villages that are missing on maps.

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Using imagery to field-verify EA maps  The disadvantages are as follows: The spatial resolution of many systems, especially low cost ones, is not sufficient for census applications; In the case of optical sensors, cloud and vegetation cover restricts image interpretation; The problem of low contrast between features — e.g., dirt roads and traditional building materials in rural areas — makes their delineation particularly difficult in developing world contexts; and Image processing requires a large amount of expertise – which may not be available at the NSO.

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Application of aerial photos for census mapping  Orthophoto maps are well-suited for dwelling unit counts and population estimation  A reliable estimate of the average number of persons per household then allows a sufficiently accurate estimate of population for census purposes  Census staff can then delineate enumeration area boundaries that include a specified number of housing units  Additional geographic features that provide the geographic reference for the enumerators can also be extracted from the photos.

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Application of aerial photos for census mapping  Implementation and institutional issues with aerial photography 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 another national agencies, most likely the mapping department or an air force reconnaissance unit.

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Application of aerial photos for census mapping  Orthophotos are Useful as a backdrop to provide a context for the display of point locations collected using GPS or digitized features such as health facilities and transport networks Today orthophotos and other remotely sensed imagery is more likely to be built into a GIS project than included as a separate map Digital orthophotos often have very high resolution, with pixel sizes on the ground in the centimeter range (usually 5-30 cm) Resampled digital orthophoto images with pixel sizes between 0.5 and 2 meters are sufficient for delineating EAs in urban areas.

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Application of aerial photos for census mapping  The advantages of aerial photos include: Data collection is faster and map data can therefore be produced much more quickly than using cartographic ground surveys; Topographic mapping using aerial photography can be less expensive than mapping using traditional surveying techniques. However, since the accuracy requirements for census maps are lower than for topographic mapping, the considerable costs are not necessarily justified if the products are used for census mapping only; and Printed aerial photos are useful in field work to provide the “bigger picture”. Field staff can see the terrain that is visible from their viewpoint in the wider context of the surrounding area.

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Application of aerial photos for census mapping  The disadvantages are as follows: Aerial photo processing requires expensive equipment and specialized expertise. Census offices therefore need to collaborate with other agencies to gain access to orthophotos, or otherwise rely on outside support; Aerial photos may be copyrights, with limited distribution rights; Aerial photos still require information on the names of features which need to be extracted from possibly outdated maps. Aerial photography does not necessarily make field work unnecessary. Most likely they will not be adequate for remote areas, although they may provide backup for hard-to-enumerate areas; Aerial photo interpretation may be difficult where features are hidden under dense vegetation or cloud cover, or where limited contrast provides no clear distinction between adjacent features (for instance, between homesteads made of natural materials and the surrounding ground); and Digital aerial photos consist of very large amounts of digital data and therefore require fairly powerful computers for display and further processing

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round Conclusion  Touch on the process of utilizing tools such as global positioning systems and remote sensing in census fieldwork, integrating them with ground-based work.  Making use of these geospatial tools can allow the NSO to focus efforts on rapidly changing areas within the country.  By this point in the census process, the NSO has completed a geodatabase of enumeration areas.  The next step is to design, print, and distribute maps from the geodatabase for use in the actual enumeration

UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round END