eModule 2 – Introduction to geographical data

Digital boundaries are often used to represent the geography of the census. The same geography can be represented in different ways under either the vector or raster data model. This is how a pair of digital boundaries are represented in a digital boundary dataset – a vector dataset. A vector dataset stores the feature as sets of points, lines and polygons. The same geographical features could also be represented in a raster dataset. A raster dataset used a regular grid of cells, each of which is the same size. Digital boundary dataset are normally supplied as vector datasets although other geographical datasets may provided as rasters. An example of this would be an aerial photographic or a scanned topographic basemap which could be used to provide context to a set of vector digital boundaries. Geographical datasets contain information about features that are located on the surface of the Earth. The Earth is shaped like a globe. For ease of use and manipulation geographical datasets are normally transformed (projected) to a flat representation. There are many different types of transformation which may lead to the final data being projected in different ways. When using UK Census data we typically use data which has been transformed using a Tranverse Mercator projection. Once a geographical dataset has been projected to a flat representation, data within that representation is located relative to a specific spatial reference system which defines among other things a local origin and units of measure. UK Census data normally uses the spatial reference system called the British National Grid. The origin of the British National Grid lies near the Isles of Scilly off the South West coast of England. Whenever we talk about the coordinates of a geographic feature within the British National Grid we are talking about the distance offset (typically in metres) in X (Easting) and Y (Northing) from this point of origin. In the above origin the coordinate of the lower left corner of the polygon is offset equally from the origin of our spatial reference system in both x and y. The actual coordinates of this point could be referred to as 100000,100000. If we were to go clockwise, right to the next point in the polygon, its coordinate might be something like 280000,100000. We are increasing the value of X but keeping Y the same. Above we have shown the same geographic feature at different scales. As we move from large to small scales, the area of space represented by 200m changes. The geography of the UK can be complicated. There are many different types of geography which are in use and which have been defined for different purposes. Geographies are also not static and therefore data linked to those different geographies may not always be comparable. Within the context of the census, geography may be represented digitally as a digital boundary dataset. The geographic relationship between 2 or more different geographies can be captured by a geographic lookup table. If you are not familier with the geography of the UK or if you need to refresh your memory then we would recommend that you take a look at eModule 1. Source.

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Last Modified: April 18, 2016 @ 7:04 pm