Regional Workshop on the 2010 World Programme on Population and Housing Censuses: International standards, contemporary technologies for census mapping

Regional Workshop on the 2010 World Programme on Population and Housing Censuses: International standards, contemporary technologies for census mapping and data processing

Minsk, Belarus, 8-12 December 2008

Statistical Analysis & Dissemination of Census Data



Statistical Analysis and Dissemination of Census Data

Outline The Power of Maps

Introduction and Example Dynamic Census Atlases

Overview & Example

Spatial Analysis Techniques Overview & Examples

Digital Geographic Data for Dissemination Overview & Cost and Benefits

Digital Data Dissemination Strategies and Users Overview of Users



Anyone or anything can be associated with a known location in the world



CHILE: HOUSING AND POPULATION CENSUS DISTRICTS 2002



Tsunami Affected Areas in Gizo, Solomon Islands



The power of maps

Maps communicate a concept or an idea. Maps are often meant to support textual information Maps appeal to the viewer’s curiosity Maps summarize large amounts of information concisely Maps can be used for description, exploration, confirmation, tabulation Maps encourage comparisons:

Between different areas on the same map: where are population densities highest?

Between different maps: is child mortality higher in the districts of province A than in province B?

same area: where and by how much do literacy rates for males and females differ in the districts?

Between maps for different time periods: did fertility rates decline since the last census?



Dynamic census atlases

Alternative to a static census atlas

Publishing of a digital map and database together with mapping software can allow users to produce custom maps of census indicators.

Normally includes digital boundary files at a lower resolution than the full census database to allow fast drawing and low disk usage

closely integrated attribute table should contain only a selected number of census indicators.

Densities and ratios that are appropriate for mapping should already be calculated.



Dynamic census atlases

The data provider should therefore provide an easy-to-use package together with the boundaries and data.

The use of that package should require minimal training and experience.

The application should be “plug-and-play”—after installation, the user should immediately be able to produce maps

Drill-down options for different geographic selections

Interactive area delineation options (e.g. select schools in a district)



A screenshot of Ukraine’s dynamics census atlas



Spatial Analysis Techniques

the main use of spatial analysis is for census products and services

Techniques include: buffering, linear interpolation, point pattern analysis, and cartograms, etc.

All offer functionality beyond standard thematic (choropleth) mapping, with many tools now available in both commercial and open-source software programs.




Some prevalent forms of spatial analysis especially useful for use with population data include: Queries Distance measurements Transformations

Buffering point-in-polygon analysis Polygon overlay analysis




Queries: Often this is the first step in an analysis, where one seeks to create

a subset of units such as populated places with certain characteristics, allowing the user to check how typical an observation is against other observations

They use a GIS program to answer simple questions posed by the user, with no changes in the database and no new data produced.

An example of a query using geocoded census data is, select all towns with a population greater than 1,000 persons. These towns can then have their attributes summarized, for instance, to measure their total fertility rates against smaller towns and villages, then map the results

The term exploratory data analysis refers to investigations of patterns and trends in data using such techniques as querying



Area delineation

E.g. Interactive determination of school districts with the same number of children in each school grade by aggregating census dissemination areas




Distance measurements Easily done with all GIS programs, using the centroids (or

center points) of cities, towns, and villages.

An analysis can be done to select villages located more than a kilometer from a school, clinic, or water source.

These can then be further analyzed using the attribute information for the populated places themselves.




Transformations

Methods of spatial analysis that use simple geometric, arithmetic or logical rules to create new datasets

Transformations can include operations that convert raster into vector data, or a stream of GPS coordinates into a route or a boundary

Of all the transformational techniques, buffering is the most well known and important




Buffering (transformation)

Involves building a new data layer by identifying all areas that are within a certain specified distance of the original.

Buffering can be performed on points, lines and polygons and can be weighted by attribute values.

Buffering can be used to model travel time, for instance, by creating a “catchment area” around a particular feature such as a school or a clinic.

This provides a measure of accessibility that can be mapped across the extent of a country.



“is near to”: Buffer Operations

• Point buffer

• Affected area

around a Hospital

• Catchment area of a

water source



Buffer Operations

• Line buffer

• How many people live near the polluted river?

• What is the area impacted by highway noise?



Buffer Operations

• Polygon buffer

• Area around a reservoir where development

should not be permitted




point-in-polygon analysis Determines whether a point lies inside or outside a

polygon. Can be used to compare geocoded village centroids lying

inside and outside hazardous areas such as tropical storm tracks or earthquake zones.

Polygon overlay analysis Involves comparison between the locations of two different

polygonal data layers. For example, the boundaries of two administrative districts

could be compared to troubleshoot errors in the field enumeration process




Spatial interpolation A spatial analysis method designed to fill in values that lie between

observations

A variety of methods including inverse-distance weighting and kriging are used to estimate the values of unsampled sites

based on Tobler’s first law that all nearby objects are more similar than distant objects

Kriging: interpolation technique for obtaining statistically unbiased estimates of spatial variation of known points such as surface elevations or yield measurements utilizing a set of control points

In kriging, the general properties of a surface are modeled to estimate the

missing parts of the surface



Example of linear interpolation creating contours



Thiessen polygons illustrated

Thiessen polygons Have the unique property that

each polygon contains only one input point (e.g. settlements), and any location within a polygon is closer to its associated point than to the point of any other polygon.

This method assumes that the values of the unsampled data are equivalent to those of the sampled points.




Areas of influence

Commuting distances: daily commuters flow




Descriptive summaries are a spatial equivalent of descriptive statistics (such as mean and standard deviation) that represent the essence of a dataset in 1 or 2 numbers

Centers of population are the two-dimensional equivalent of a statistical mean and are often used to display the center of population using the weighted average of x and y coordinates of populated points

Point pattern or cluster analysis regards the distribution of points in space irrespective of their actual locations to determine whether patterns are random, clustered, or dispersed

hot spots are where high values are surrounded by high values, or cold spots, where low values are surrounded by low values. These are particularly useful for identifying populations at risk as well.




Cartograms sometimes used to display

census results

The areas of the original polygons are expanded or contracted based on their attribute values such as population size or voting habits



Modelling: smoothing

Evolution of the population beetwen two censuses



Digital Geographic Data for Dissemination

Demand for digital databases that consist of extractions of the census agency’s digital geographic master database will only

increase Census data are an important input in policy planning and

academic analysis in many fields.

Health service provision, educational resource allocation, design of utilities and infrastructure, and electoral planning are

some applications where government agencies require spatially referenced small area population statistics.

Commercial users employ such data for marketing applications and location decisions.



Digital Geographic Data for Dissemination

Benefits and costs Benefits: Unsurpassed detail and precision, the potential

use of census data in numerous applications--especially when overlaid on other geographic data such as terrain, and the relative ease of management and storage of thousands of units

Costs: expense in processing and data management, possible data disclosure issues, and quality control;costs of metadata production should be factored into the equation as well



Digital Data Dissemination Strategies and Users

The wide range of potential users of disaggregated census data means that the NSO needs to pursue a multi-leveled digital data dissemination strategy.

Broadly, we can distinguish between the following types of users:

Advanced GIS users Computer literate users Novice users




Advanced GIS users work easily with large datasets and can use ftp to access them

Require extensive metadata. Sometimes called data extractors or “power users”

They will want access to spatial and attribute information in a comprehensive digital geographic format

The census office needs to supply comprehensive documentation on the geographic parameters used for the geographic database as well as on the individual census variables

The spatial information will be distributed in an open geographic format that can be easily converted into any number of commercial GIS formats




Computer literate users Government, commercial or private sector users who want to be able to

browse the thematic information in a census database spatially.

Want to produce thematic maps and thus need to be able to perform simple manipulation of cartographic parameters.

Simple analytical functions such as aggregation of census units to custom-designed regions should also be possible.

This group of users is best served with a comprehensive, pre-packaged application that is designed for a commercial or freely available desktop mapping package.

Documentation requirements are somewhat smaller, since the users are unlikely to change the geographic parameters of the database or perform more advanced GIS operations.




Novice users Largely want to view pre-prepared maps on a computer and

perhaps perform some basic queries

Best data distribution strategy is often to produce a self-contained digital census atlas

This atlas could consists of a series of static map images, for example, in the form of a slide show

Or it could be a very simple mapping interface with pre-designed map views that allow basic queries

Both, static maps and a simple map interface, can be made accessible through the Internet



GRACIAS POR SU ATENCIÓN

Documents

Regional Workshop on the 2010 World Programme on Population and Housing Censuses: International standards, contemporary technologies for census mapping