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 (multipath 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
EA boundaries delineated atop
a panchromatic satellite image
 With the provisional EA
boundaries superimposed on
R/S imagery population
settlements can be quickly
located and priority areas
identified.
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 2001.

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
0.5 m
1m
2m
5m
10 m
UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round
Very high spatial resolution civilian satellite
remote sensing products
•Product
•Company
•Launch
•Mode
•Pixel Size at
nadir
•Height (km)
•Quickbird
•Digital Globe
•2001
•Pan/4ms
•0.61/2.44
•450
•Ikonos 2
•GeoEye
•1999
•Pan/4ms
•0.82/3.28
•680
•OrbView 3
•OrbImage
•2003
•Pan/4ms
•1.0/4.0
•470
•Spot 5
•SpotImage
•2002
•Pan/4ms
•5(2.5)/10
•830
•Cartosat1
•NASDA,
Japan
•2004
•Pan
•2.5
•617
•Cartosat2
•NASDA,
Japan
•2004/5
•Pan
•1
•630
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
UNSD-CELADE Regional Workshop on Census Cartography for the 2010 Latin America’s census round