Census Maps, Uses, Elements and Types

Census functionaries need to have the basic knowledge of maps, its interpretations and how to use maps to identify enumeration areas and fetch EA frame before commencement of census exercise.

Find details of all maps related information for a successful population and housing census exercise by enumerators and other census functionaries.

What is a Map? 

A map is a representation of either the entire earth surface or its part in reduced form, showing natural and man-made features existing on it. 

A map may be described as a carefully, systematically designed visual display for recording, calculating, displaying, analysing and understanding the interrelationship of a given spatial information. A map is drawn to scale.

Sketch: A sketch on the other hand is a non-systematic graphic representation of features. It is usually not drawn to scale. Sketches are useful when maps are not available. 

Elements of Census Maps

Digital Map

A map may be in hard copy or Digital Format.

Digital maps are compiled as a virtual image on the PDA and can be zoomed in or out to desired scale as applicable. This is the main advantage of the Digital Map over the hard copy map.

Digital Map are represented in both Raster and Vector forms. Raster means Satellite Imagery and Vector format (uses symbols)  - line, points and polygons.

Satellite Imagery

These are remotely acquired data of parts of the earth surface (ground features) by space born satellites. They are data based on reflected or emitted electromagnetic radiation collected from orbit that can be used to create photograph-like representations.

Uses of Satellite Imagery

• It provides an overview of the entire area it covers.
• It aids total coverage as all buildings and other features serving as the EA boundary can be clearly identified.
• It helps non-map experts to easily identified map features unlike vector maps where features are represented with symbols.
• Satellite Imagery is used as a backdrop of the EA boundary and other Census features.

Interpretation of Satellite Image

It's important for enumerators and other census functionaries to have a basic knowledge of how to recognise features on the satellite imagery.

The Census basic image interpretation principles are colour or tone, shape, association and size.

1. Colour or Tone: Old buildings with rusty corrugated roofs will have dull brownish colour while newly constructed buildings with new roofs will have brighter colours depending on the roof type. New excavations will have a bright appearance, etc. Tarred roads will have grey colour and vegetation green. Swampy areas with overgrown vegetation will have a darker greenish colour.

2. Shape: Tarred roads appear with straight edges while footpaths and untarred roads have rough edges. All natural features such as forest, streams, etc will have irregular edges/shapes while man-made features such as buildings, roads will have regular edges/shapes.

3. Association: An identified feature can help to interpret other naturally associated features. For instance, the presence of a hub of vehicles points to a motor or car park. The presence of boats in the imagery gives an idea of a harbour. A road crossing a river means there is a bridge. A wide playground with organised buildings depicts a school, etc.

4. Size: Size of objects in an image is a function of scale. It is important to assess the size of a target relative to other objects in a scene, as well as the absolute size, to aid in the interpretation of that target. A quick approximation of target size can aid direct interpretation of appropriate result more quickly. E.g. large buildings such factories or warehouses would suggest commercial property, whereas small buildings will indicates residential use.

Vector Maps

Vector Maps are use of points, lines and polygons to represent physical or imagery census entities in the Enumeration Geo-Database. The lines, polygons and points are tied to the Satellite imagery as an overlay.

Vector Representations

• Polygon - Boundaries and Features that span over space (State, LGA, ward, SA & EA, water bodies, crisis areas, etc).
• Line  - Line features (roads, foot paths, rivers, etc).
• Points - Buildings and other related features (buildings, important features, corner buildings, etc).
• Vector maps are superimposed on the Satellite Imagery as a Single Digital Map for census exercise.

Types of Maps for Census

1. Local Government Area Maps (LGA Maps): It is the administrative map fir each LGA. It is the seamless array of all the EAs for entire LGA covering every inch of land without overlap or gaps. It is to aid planning for Census to allocate both human and material resources on LGA basis as well as monitoring of Enumeration progress during head count.

The feature layers on the LGA Maps include:

• Satellite Imagery
• Enumeration Area boundary with displayed ID at specific zoom level.
• Supervisory Area Boundary with displayed ID at specific zoom level.
• INEC Registration Area/Ward with displayed ID at specific level.
• Important features such as schools, hospital, petrol stations, etc to aid location identification.
• Drainages (rivers, water bodies, canals, etc).
• Roads well annotated
• Building: All buildings identified during the demarcation exercise.

2. Supervisory Area (SA) Maps: This is the Supervisors' administrative map covering a specific area with a number of EAs allocated to a Supervisor for monitoring. It will have the total number of EAs within the SA clearly identified with their boundaries and IDs displayed on the map.

The SA map is a 'Zoom In' from the LGA map. This will aid the Supervisor in seeing the neighbouring SAs/EAs as well as aid each Enumerator within his/her Supervisory Area to identify his/her boundary of Enumeration. It will have all the feature layers of EA, SA boundaries, important features, drainages, roads, footpaths and all buildings within the EA. All the features are well annotated or coded where applicable. All LGA and ward boundaries are also shown where applicable.

3. Enumeration Area (EA) Map: This is the map covering the area assigned to each Enumerator for the census exercise. It shows the coverage of the EA with the EA boundary clearly delineated and the EA ID displayed on the EA polygon. It shows all buildings within the EA as a point resting on the Building's Satellite Imagery feature.

A building code will also appear by the side of this point in numeric nos e.g. 1, 2, .......n, where n is the last code of the building within the EA and means the total number of buildings within the EA.

The EA map is a 'Zoom' in of the SA map and therefore the Enumerator can easily identify the neighbouring EAs to avoid moving out of his/her EA during enumeration.

Map Orientation

The orientation of a map is the relationship between the directions on the map and the corresponding compass (north, east, West or south) directions on ground. It is important to understand map orientation because aligning a map to features on ground is dependent on which direction is faced. 

For an Enumerator to conclude that a feature identified on the map is on the right or left side on ground is dependent on the direction he/she is facing. 

The north of a georeferenced map is always at the top of the map. Therefore, Enumerators should align their maps 'fetched' on the PDA with features that they can identify on the ground.

Map Navigation

Digital maps shall be used to identify and navigate to the Enumerators' area of assignment. The Digital Map for this Population and Housing Census is running on a customised application 'Pad System' which has the capability to help Enumerators identify their area of assignment easily and also has the tool to direct the Enumerator to the area of assignment.

Map Updating

This is the process of adding information about new features that are not found or named on the map such as new buildings, floating population, etc and also deleting same that are no longer in existence. 

Map updating will be done during the Building and Household Listing. All new buildings and other target population areas shall be added to the Database and all non-existing features shall be annotated as such.

Digital Maps for PHC (Population and HousinG census)

The outcome of the Enumeration Area Demarcation for the Population and Housing Census produces the Enumeration Area Geo-database, the National EAs frame table, Locality list, hard to count and LGA EA administrative maps.

• 1. Enumeration Area  Geo-databse with the following feature classes: Enumeration Area, Supervisory Area, Local Government Area, States, Locality, Ward/RA, buildings, roads and drainages.
• 2. A National Frame table of all EAs carved with each EA having its own unique identify.
• 3. Locality List on LGA basis
• 4. Hard to Count EA identification map and table/LGA basis.
• 5. LGA EA Administrative Maps.

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