L7 - Raster Algorithms L7 – Raster Algorithms NGEN06(TEK230) – Algorithms in Geographical Information Systems

L7 - Raster Algorithms

L7 – Raster Algorithms

NGEN06(TEK230) –

Algorithms in Geographical Information Systems


Background

Store and analyse the geographic information

Raster data or Vector data.

Raster data have been common in analysis in physical geography.

Aerial and satellite images within GIS

Continuous surface presentations


Aim

The main objective is to familiarize the students with some raster algorithms. After this lecture the student should be able to write a

minor program that treat raster data in one of the areas cost analysis, shaded relief, viewshed and resampling.


Content

1. Introduction

2. Local and regional operators

3. Morphological operators

4. Shaded relief

5. Resampling

6. Flow analysis in hydrology


Example of raster data and raster analysis

1. Digital mapsCost analysis, location analysis, overlay analysis and generalisation (e.g. by morphological operators)

2. DTMs and DEMsViewshed analysis, flow analysis, and shaded relief .

3. Images (most often aerial or satellite images)

Resampling, image classification and information extraction.


Example of raster analysis

1) Morphological operators

This is a basic method for digital maps. Several manipulations and analysis of raster maps are based on morphological operators. (shrink and expand) L9 genralization

2) Shaded relief

A shaded layer is created (based on the DEM) for a fictitious sun; this layer is then added to a map to enhance the feeling of topography.

3) Resampling of images

To use e.g. an ortophoto in a geographic analysis it must be geocoded. Geocoding - resampling of the image to the new geometry.

4) Flow analysis in hydrology

All above are regional operations


Vector vs. Raster

Distances, area of polygon, point-in-polygon are all fairly simple to perform with raster data.

Result is often worse than in the vector case since the resolution of the raster data is not that good

Means that the path between two pixels with the lowest total cost (sum of the pixel values for the path) is found.Raster version of Dijkstra’s algorithm can be used.

Cost analysis for raster maps

Raster data is often used for locational analyses. For example, all buildings that are closed to a lake, close to a road and far from industries are sought.


Local and regional operations

Kernel filter


Morphological operators


Shaded relief

Enhance the feeling of topography in a map.


Shaded relief


Computing the shaded relief


Approximating the partial derivativesSteepest gradient approach


Approximating the partial derivatives Sobel filter


Resampling

Original gridTransformed grid


Resampling method

It is important to state what kind of raster data that is used and the application of the resampled data.

If the raster data is a digital map (or contain any type of data in nominal or ordinal scale) it is not allowed to do calculations on the raster data values. A recommended resampling method is then nearest neighbour.

If the data is an image (which contain continuous data that is in ratio or interval scale ) it is allowed to do calculations on the raster data values


Nearest neighbour resampling

Original gridTransformed grid


Inverse distance resampling


Flow analysis in hydrology


Drainage Direction – 2 ways

• Simplified: In a matrix of 9 cells, the water from the centre cell flows to the neighboring cell with the lowest elevation

• OR, for more realistic algorithms, is distributed proportionally to a number of neighboring cells with lower elevation

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L7 - Raster AlgorithmsSurface flow algorithms over digital elevation models

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Single flow direction algorithm SFD

Multiple flow direction algorithm MFD


Topographic form


A convex surface (flow is split)

Multiple flow direction algorithm

Form based flow direction algorithm


A concave surface – one directional flow

Multiple flow direction algorithm

Form based flow direction algorithm


Break line

Flat area

Sink

Artefacts in real world data

Stream Network

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Flow Accumulation Grid.

Area draining in to a grid cell

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Stream Network for 10 cell Threshold Drainage Area

Watershed Draining to Outlet

Estimation of flow accumulation

Why we need to create new flow distribution algorithm ?

Problems related to sinks, man made structures and flat areas.

An empirical equation is used to estimate flow distribution in most flow distribution Algorithms.

The flow is distributed to some or all down slope cells, no influence from upslope cells.

Concave verses convex form.

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Triangular form-based multiple flow distribution algorithm (TFM)

Area of one triangle =1/8 area of grid cell

Triangles has a constant slope

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Drainage area estimation on standard surfaces

Mathematical

estimation

TFM

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ConcaveConvex Plane Saddle


TFM

Documents

L7 - Raster Algorithms L7 – Raster Algorithms NGEN06(TEK230) – Algorithms in Geographical Information Systems