Vector vs Raster
Ordnance Survey Ireland data is supplied in both Vector and Raster format.
In both cases the data is geo-referenced. If you are working in a GIS application the mapping is positioned relative to it’s place on the Irish Grid.
Vector data consists of a series of nodes stored as x,y co-ordinates that define line segments. These line segments when joined form map features such as roads, buildings and rivers.
Vector datasets have all features coded or multi-coded to represent topographical features. Available in both the Irish National Grid and Irish Transverse Mercator.
This vector data can be displayed at any scale with individual layers capable of being omitted or individually shown.
The current OSI datasets consists of 179 levels.
Supplied in DXF,DWG and NTF and SHP format.
Note on SHP files:
At the moment features are stored under different headings and these will be standardised throughout the industry. See the examples below of 3 variations.
System 1 is the current OSi standard.
System 1 System 2 System 3
Example: Spot Height. Description Symbol Point Point
Example: Road. Description Line Polyline Edge
Example: House. Description Area Polygon Face
Example of layers turned on and off.
Screen shot shows 1:50000 Discovery Series data in Vector format, all layers on.
Using the same screen shot, only two layers are shown. The layers are Antiquities and National Rail.
As mentioned, the current OSI datasets consists of 179 levels.
They may be grouped to form a standard 35 levels as required. Levels include Antiquities, Area, Barony, Boundary, Building, Contour, County, Glasshouses, High Water Mark, Low Water Mark, National Rail, Parish, Roads, Road Centre, Text, Townlands, Ward, Water.
Vector map data can be used to dynamically present
thematic information, by incorporating
Population Census information or Environmental data etc.,
The information below is supplied by C.S.O.
Raster data provides a map image formed by a matrix of pixels arranged in rows coloums, which can be displayed on a computer screen.
Scanned image at 400 or 500 d.p.i, or image generated from vector data at 300 dpi, reproducing data on published map sites.
Data format :Supplied in Tiff (packbits) form. Historical Coloured 6 inch mapping is available in Jpeg.
Raster data is made up of cells called pixels. You may already be familiar with
pixels if you use a digital camera.
I have zoomed in on the image and you can see the individual cells or pixels.
However the image is not clear at all.
I have zoomed back out and now you can see the image below shows a raster extract of the 1:50000 discovery series.
Raster maps are often used as a background or base map.The image below shows an extract used as a background map with an Ortho Photo overlaid in the South West corner.
Raster is not as intelligent as Vector. It is more useful as a display map for brochures. internet and power point presentations.
Vector data can be manipulated.
Layers can be turned on and off.
Data can be edited or deleted. Additional data can be added in.
Raster data is usually used as a background map.
You cannot edit the data.
You can add overlays as in the Ortho photo on the image above.