Home / Summaries / An introduction to geographical information systems / raster-vector-entity

Spatial data modelling - Entity definition

48 important questions on Spatial data modelling - Entity definition

There are a number of problems associated with simplifying the complexities of the real world into five basic, two-dimensional entity types Why?

because the real world is not static. forests grow and are felle; rivers flood and change their course; and cities expand and declineer

The dynamic nature of the world poses two problems for the entity-definition fase of a GIS project. which ones ?

1.how to select the entity type that provides the most approriate representation for the feature being modelled.is it the best to represent a forest as a collection of points, or as an area
2. is how to represent changes over time. a forest originele represented as an area, may decline as trees die or are felled until it is only a dispersed group of trees that are better represented using points.

The definition of entity types for real-world features is also hamerde by the fact that many real-world features simply do not fit into the categories of enitities available. give an example.

an area of natural woodland does not have a clear boundary as there is normally transistion zone where trees are interspersed with vegetation from a neighbouring habitat type.

At present there are 2 main ways in which computers can handle and display spatial entities, which one ?

these are the raster and the vector approaches.

How is the raster world used ?

in the raster world individual cells are used as the building blocks for creating images of point, line, area, network and surface entities.

What is the basic building block in the raster world?

is the individual grid cell, and the shape and character of an entity is created by the grouping of cells.

What does a vector spatial model doing ?

this uses a two-dimensionaal cartesian x,y co-ordinates to store the shape of a spatial entity.

What is the building block in the vector world ?

in the vector world the point is the basic building block from which all spatial entities are constructed.

What is the simplest spatial entity ?

the point is represented by a single x,y co-ordinate pair.

What are line and area entities ?

line and area entities are constructen by connecting a series of points into cahiers and polygons

What is complex when using the vector approach?

selecting the appropriate number of points to conctruct an entity is one of the major dilemma's when using the vector approach.

In the raster world a range of different methods is used to encode a spatial entity for storage an representation in the computer. which ones ?

a.entity model
b.cell values
c.file structure

How can duplicated data be avoided ?

this can be avoided by making use of adjacent polygons sharing common co-ordinate pairs.

What is the main advantage of TIN?

the efficiency of data storage, since only a minimum number of significant points need to be stored to reproduce a surface.

Describe the term topology

topology is the term used to describe the geometric characteristics of ojbects which do not change under transformations such as stretching or bending and are independent of any co-ordinate system.

Topology as it relates to spatial data, consists of three elements which one ?

adjacency, containment and connectivity.
adjacency and containment describe the geometric relationships that exist between area features. areas can be described as being adjacent when they share a common boundary.

There are 2 types of geometric models that are widely used to store spatial data in a GIS. Which are those models?

Vector model and the raster model

The vector model uses three entity types to represent spatial phenomena:

points, lines and areas (polygons)

What is a problem with DEM where the terrain is varies or complex?

Choosing an appropriate resolution is difficult

Fine resolution raster will accurately model the mountains in the west, but a high data redundancy will be experienced in the flat areas in the east
If you choose a low-resolution raster such as might be suitable for the flat eastern part of the area, the detail of the mountains to the west will be lost.

The choice for a particular entity type depends on:

scale, context and purposes

What is the problem with Voxel?

It is unable to record topological information

Within the computer, each of the entity types consists of a series of one or more?

coordinate points. for example, a line is a collection of related points, and a polygon is a collection of related lines.

Within GIS system, the spatial entities of a vector model are typically ?

contained in an attribute table.

What can be showed via a single X,Y coordinate ?

a point entity is a zero-dimensional representation of an entity.

Whether an entity is too small depends on the scale on which it should be represented give an example ?

a capital would be represented as a point on a world map with countries and capitals, but the same capital would be represented as a polygon on a national or regional map

Give some descriptions of line entities?

-a line has no width, only length
-it is stored as two or more connected coordinates
-the starting and ending coordinates are called "nodes" and the inbeelden coordinates "vertexes"
-a synonym for line in GIS is arc

What is a polygon?

A polygon is a 2 dimensional representation of an area entity. a polygon is defined by the lines that make up its boundary.

This method of representing areas has many disadvantages and causes problems. Which one ?

-lines between adjacent polygons must be digitized and stored twice
-there is no neighbourhood information
-islands are impossible except as purely graphical constructions
- there are no easy ways to check if topology of the boundary is correct or incomplete or makes topologically inadmissible loops.

What is an important asset of the vector model?

the potential to store spatial relations between entities.

To describe the position of an entity in relation to other entities, topologie consists of three elements which one ?

adjacency, containment and connectivity

How is the raster model organized ?

raster data are organized as a matrix of grid cells or pixels, with spatial position impliciet in the ordering of the pixels.

What is is continuous raster ?

a continuous raster contains real values which are expressed on continuous scale. continuous raster datasets do not have an attribute table; the cell values are stored together with the spatial location of the cell.

What does a discrete raster ?

uses a limited number of categories or classes.discrete raster datasets do have an attributen table which shows how many grids cells fall within each category

When is a raster structure especially helpful ?

when a spatial analysis demands simultaneous processing of various datasets.

What is a typical feature of the raster model ?

is that a grid or raster cell can contain only one value, e.g. the average temperature in this location.

Raster structures take relatively much storage capacity compared to the vector model is this true ?

yes, they store in each cell in the matrix regardeer of whether it is a feature or simply empty space

In GIS the term pixel refers to

a grid cell

Pixel or cell size refers to resolution which is

defined as the smallest unit of information available in an image or raster map.
this is the smalst element of a display device that can be independent assignet attributes such as colour.

Which two discriptors are needed to describe spatial data and what does each of these discriptors describe?

Spatial data (location) and attributes (the characteristics of the object described).

Describe how point, line and surface entitities are represented in a vector model and a raster model, respectively.

The vector approach uses points, lines and areas to represent objects that can be considered points (e.g. trees, poles, houses, cities, etc), lines (roads, railway tracks, rivers, borders, etc.) and areas (vegetation types, neighbourhoods, rivers, farm land, etc.).
The raster approach always uses a (combination of) raster cells to represent points, lines and areas.

Describe the relationship between scale and the use of an entity type to represent entities.

An object that is represented as an area on a large scale, can be represented by a line (e.g. river) or point (e.g. city) on a small scale.

Why is the vector data structure often used to implement discrete perceived spatial phenomena?

A vector data structure consists of points, lines and polygons. These are all objects with a discrete border (i.e. a sharp border between the objects and the environment). A vector structure is therefore very useful to represent discrete phenomena such as houses, roads, rivers, etc.

Why is the vector data structure not very suitable to implement continuous perceived spatial phenomena?

A continuous phenomenon is a property/feature that has a value on every location. An example is the average annual rainfall. Each location has its own characteristic average annual rainfall value. So, rainfall is not an entity, but an (omnipresent) feature. It is very difficult to present such a feature with the points, lines and polygons used in the vector data structure. The only way to do this is by using iso-contours, i.e. nested polygons which connect the points which have an equal value (e.g. for rainfall or height).

Give an example of how a continuous phenomenon can be implemented in a vector data structure.

see previous answer

Give an example of how a discrete phenomenon can be implemented in raster data structure.

This phenomenon will be represented by one or more grid cells. For example, a house may be represented by one grid cell, a road by a series of connected grid cells and a meadow by a block of identical grid cells.

Imagine you want to implement discrete phenomena in a raster data structure. Try to describe the relationship between raster cell size and the size of the discrete phenomena.

The raster cell size will determine the accuracy of the map. When the discrete objects are relatively large in comparison to the raster cell size, the map may be quite accurate. However, when the raster cells are relatively large, the object may be distorted, because the raster cells will determine the appearance of the object on the raster map. If the raster cells are much larger than the objects, it may happen that more than one discrete object falls into one raster cell. This can result in serious inaccuracies.

A Digital Elevation Model (DEM) is often called a 2.5D model. Why is it not called a 3D model?

This is because a 3D model is a real model in three dimensions, like a scale model. However, a paper DEM only has two dimensions. The third dimension is visualized using colours or isocontours. To emphasize the difference, such a DEM is often referred to as a 2.5D model.

Why is topology important for spatial analysis?

Because topology tells the computer the relationships between the elements of a vector data structure: the points, lines and polygons. Topology establishes how points, lines and polygons are related, e.g. whether two lines intersect or cross.

The question on the page originate from the summary of the following study material:

An introduction to geographical information systems | 9780273722595 | Ian Heywood, et al

View summary

A unique study and practice tool
Never study anything twice again
Get the grades you hope for
100% sure, 100% understanding

Remember faster, study better. Scientifically proven.