Presentation on theme: "Data Models There are 3 parts to a GIS: –GUI –Tools –Data Management System May be distributed on separate machines connected by a network We will look."— Presentation transcript:
Data Models There are 3 parts to a GIS: –GUI –Tools –Data Management System May be distributed on separate machines connected by a network We will look today at the different ways in which the data are stored within a GIS
Levels Of Abstraction Can identify four levels of abstraction: –Reality – i.e. the real world –Conceptual model - a human-orientated, partially structured model of selected objects and processes relevant to a particular problem domain. –Logical model – an implementation-independent, but implementation-orientated representation of reality. It is often represented as a diagram showing the selected objects and relationships between them. –Physical model – a physical model describes the exact files or database tables used to store the data, etc. It is specific to a particular implementation.
Conceptual Models Can identify three conceptualisations of space: –Field-based – attributes can be thought of as varying continuously from place to place (e.g. precipitation). Can be 2-D or 3-D (e.g. air pollution). –Object-based – features can be thought of as discrete entities or objects. Can be large or small, physical or counties, and con contain other objects. –Networks – object-based, but emphasis is on the interaction between objects along pathways.
Logical Models The term spatial (or geographical) data model is used to describe how data are organised within a GIS. The two main types are: –Raster. Study are is divided into regular cells (usually rectangular). Often used to model field data, but do not actually form a continuous surface – sample points. –Vector. Geometric primitives (i.e. points, lines, polygons) are used to represent objects. Different phenomena are modelled as layers. In a raster model each layer represents a variable attribute; in a vector model each layer is usually a particular type of object.
Conceptual-Logical Relationships Field data are normally modelled using a raster, whilst object-based conceptualisations are normally modelled using a vector model. However, field data can be modelled using a vector model – e.g. contour lines, or using a triangulated irregular network (TIN). Raster models can be used to model objects by assigning an object identifier to each cell which can be joined to an attribute table.
Physical Models A physical data model is the specific implementation of a logical model – i.e. how the data are actually stored within the computer. The term data structure is sometimes used to describe how the data are organised within the computer. Before we look at some specific details, it is useful to look briefly at some more general considerations of data storage.
Data Storage Considerations The two main considerations relate to: –Space –Time There is usually a tradeoff between minimising the space required to access the data and maximising the speed at which it can be accessed.
Space Digital information is stored in a computer as binary digits (or bits), each of which can have a value of 0 or 1. A byte is a group of 8 bits. Bytes are sometimes in groups of 4 referred to as a word. Computer storage is usually measured in bytes. A kilobyte is 1024 (i.e. 2 10 or approximately 10 3 ) bytes. A megabyte is 1 million (i.e. 10 6 ) bytes, a gigabyte is 1 billion (i.e. 10 9 ) bytes, and a terrabyte is a million million (i.e. 10 12 ) bytes.
Search Time (1) Data on a particular entity (e.g. a person, an area, an object) are normally stored together to form a record with a unique identifier. A set of records are usually stored in a named storage known as a file. The time taken to find a specific record depends upon how the file is organised. Simple sequential files are very inefficient – average of (n+1)/2 reads. Direct access files speed up searches – i.e. can jump straight to a record if you know its record number.
Search Time (2) There are various ways to identify a record number in an index file: –Binary search. Records must be sequenced by their key field. –Hash addressing. An algorithm is used to translate key field values into record numbers (or ‘buckets’). Not necessarily a unique bucket for each key.
Search Time (3) Efficiency can be improved using an index file containing just record numbers and key fields. Further enhancements include: –Sparse index – might use every 10th record –Secondary index – can be used to identify records according to a second criteria (e.g. area of residence) Pointers are a common device in computing. Could, for example, be used to create a linked list (e.g. of people with a particular characteristic).
Raster Data Models (1) Raster data for several layers could be stored in various ways: –By location – i.e. list all the attributes for cell 1, then cell 2, etc. –By coverage – i.e. all the cells for coverage (or layer) 1, then coverage 2, etc. –By binary coverage – all cells having attribute 1 in coverage 1 saved as Boolean 1, then all cells having attribute 2 in coverage 1, etc., repeated then for coverage 2. –By data value – location of all cells having attribute 1 in coverage 1 saved as x,y, then attribute 2 coverage 1, etc.
Raster Data Models (2) Coding method affects: –Ease of edits. –Storage space – binary requires more numbers, but may require less space because each number is only 1 bit – integers require either 8 bits (if <256) or 32 bits. –Number of files required. Problems: –Data redundancy –Storage space excessive
Data Compaction Various approaches have been used to reduce storage requirements: –Run Length Encoding –Block Coding –Chain Coding –Quadtrees –Wavelet Compression – e.g. MrSID (Multiresolution Seamless Image Database). This can reduce the space required to about 2 per cent of the original. However, wavelet compression is lossy.
(26 numbers : 0,13,1,5,0,5,1,6,0,5,1,5,0,6,1,3,0,7,1,3,0,7,1,2,0,33) Run Length Encoding
Vector Data Models Real world objects are modelled in vector mode using geometric primitives (i.e. points, lines and polygons). Field data can be also be modelled using isolines or TINs, but these introduce further issues so we will ignore them for present. Features that can be modelled as points have very simple data structures: each record can contain an x and y coordinate, and multiple attribute fields. x1x1 y1y1 a1a1 b1b1 c1c1 x2x2 y2y2 a2a2 b2b2 c2c2 x3x3 y3y3 ………
Lines And Polygons Lines, polylines and polygons are more complex because each object requires more than one x,y coordinate pair. Also, the number of x,y coordinate pairs is variable. For polygons, one could check whether an x,y coordinate pair completes a loop. However, it is safer to use a special code to mark the end of the spatial definition. x1x1 y1y1 abc …… xnxn ynyn -12345
Attribute Data Attribute data is also more complex for lines and polygons. Could record the attributes for each coordinate pair, but would create a lot of data redundancy. Would also be very difficult to edit. A common solution is to store the attribute data in a separate file and link it to the locational data using a relational join. We will explore database structures next day. For present we will focus issues associated with the locational data.
Spaghetti Data Structures The visual appearance of a map could be captured by digitising lines and polygons in a random sequence without any additional information about which lines connect to which, or which polygons share common boundaries. This is akin to 'tracing' the lines on the map using a digitiser until they have all been digitised. This information could be used to reconstruct the map as it might be drawn by a cartographer. Although adequate for CAD or CAC, it is inadequate for most GIS purposes – e.g. polygon features not defined. Sometimes used for data distribution.
Arc/Node Structures(1) The DIME system developed in the 1960s was a step forward. It was the first to use an arc/node structure. A node is where two or more lines join. An arc is a section of line running between nodes. Each arc is made up from straight line segments running between adjoining points (or vertices).
Arc/Node Structures(2) Arc/node structures allow the data to be stored hierarchically. Polygons can be defined as a series of arcs. Arcs can be defined as a series of segments. The different types of data can be stored in separate files, linked together by pointers.
Arc/Node Structures(3) Arc/node structures provide several advantages: Arc between adjoining polygons only need to be digitised once. –Reduces data redundancy –Eliminates sliver lines Editing is simplified –To move a point we just need to adjust its coordinates in the points file. –To delete a point we remove the reference to it in the arcs file –To add a point we add its details to the end of the points file (no resorting) and insert a pointer at the right place in the arcs file.
Topological Data Structures(1) Further refinements were introduced in the 1980s with the introduction of TIGER files by the US Census. These added explicit topological information (e.g. the polygons on either side of an arc; the beginning and end nodes of each arc).
Topological Data Structures(2) Only require an arcs file – one can reconstruct the polygons from the topological information. ArcStartEndLeftRight 1n1n2AB 2n2n1OB 3n1n2OA Polygon B is made up from arcs 1 and 2. B is to the right of both. Nodes n1 and n2 specify the sequence in which they need to be joined.
Topological Data Structures(3) The topological information may be used to make consistency checks. For example, the coordinates of nodes can be checked for unsnapped nodes. If two arcs have the same nodes at both ends, system can check if this is because one arc was digitised twice, or they are two arcs forming a polygon. Can do lots of other checks. Data passing the checks are said to be topologically clean.
Topological Data Structures(4) Topological structures facilitate easy editing. For example, to merge the two polygons to form a new one C, remove the record for arc 1, and substitute C for A or B in the other records: ArcStartEndLeftRight 2n2n1OC 3n1n2OC
Space Considerations Vector models generally require less space than raster models, but space may be a consideration. Each X and Y coordinate generally requires 2 bytes (more if they are larger than 65535). Can reduce using relative addressing – i.e. express as offset from a local origin.