topographic correction of satellite images for improved lulc classification in alpine areas

7/27/2019 Topographic Correction of Satellite Images for Improved LULC Classification in Alpine Areas

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Grazer Schriften der Geographie und Raumforschung Band 45/2010

187

Topographic correction of satellite images for

improved LULC classification in alpine areasP. FrederZ_GIS Centre for Geoinformatics, Salzburg, Austria

pp. 187 - 194

Abstract

Automated land use / land cover (LULC) classification is hampered in areas of high relief due to shadowing effects,

which results in different values for one and the same land cover class. The purpose of this study was to reduce the

topographic effect of Landsat TM images in a mountainous study area in Tibet. Different methods of topographic

normalization (cosine correction, Minnaert correction, C-correction and statistic-empirical correction) were evaluated

and compared visually and statistically concerning quality and usability in order to improve the following LULC clas-

sification.

KEY WORDS: topographic correction, land use / land cover (LULC) classification, Landsat Thematic Mapper (TM)


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geo10th International Symposium on High Mountain Remote Sensing Cartography

1. Introduction

The operational use of remote sensing data is often limi-

ted due to sensor variation, atmospheric effects as wellas topographically induced illumination effects (Ekstrand

1996, Twele and Erasmi 2005). Topographic normalization

is therefore, especially in rough terrain, important for im-

proving analysis of remote sensing data (e. g. image classi-

fication). Although numerous topographic normalization

methods have been proposed in the past, none of them

has been found to be universally applicable, and therefore

topographic normalization is still a pre-processing issue

rarely used.

The intensity of illumination on the surface depends on

the orientation of the surface in respect to the sun. Diffe-

rent slope and aspect angles are inducing variable illumi-nation angles and thus diverse reflection values. Areas of

high relief therefore show high radiometric variation. De-

pending on topography reflection values within one land

cover type can vary a lot. The illumination variations result

in lower reflection values in the shadow and higher values

in the sun for the same land cover class. Hence, reflection

values of different land cover types in equal conditions of

illumination can be more similar than within one land co-

ver type in shadow and sun, leading to problems in image

segmentation and possible misclassifications (Twele and

Erasmi 2005, Civco 1989). Topographic normalization me-

thods try to compensate for the topographically induced

illumination variations in advance.

2. Study area

The study area, which was defined within the project

BrahmaTWinn, is located as part of the Brahmaputra (in

Tibet called Yarlung Tsangpo) catchment in Tibet (China)

and represents the catchment of the Lhasa River. The ma-

jor part of the area is situated in the prefecture-level city

Lhasa and a minor part in the prefecture Naqu (Fig. 1). The

total area is about 33.000 km2. The mountainous study

Figure 1: study area catchment of the Lhasa River in Tibet (Chi-na).

area is characterised by steep slopes and rugged terrain

with elevations ranging from 3.500 to more than 7.000

meters. These conditions create significant shadowing ef-

fects in the data.

The vegetation in the study area is mainly alpine grass-

land, minor parts are covered by shrubland, pasture, bu-

shland, arable land and a very small part by forest. A rela-

tively high portion of the land cover can be attributed to

non-vegetated areas, mainly bare ground, ice and snow.

3. Topographic correction methods

The simplest method for compensating the topographic

induced variable illumination is building of band ratios

wherefore no additional data is required. It is based on the

assumption that the relative topographic effect is similarin all bands and the quotient of two bands can compen-

sate for this. This method does not account for the diffuse

irradiance, which depends on each band, and therefore

can only partly compensate the topographic effect, pro-

vided that the atmospheric path radiance is eliminated in

advance (Ekstrand 1996). A further disadvantage in terms

of multispectral classification is the loss of spectral resolu-

tion when using band ratios (Riao et al. 2003).

Real topographic correction methods try to model the illu-

mination characteristics of a horizontal surface by means

of a DEM. For this purpose it is required to calculate the

local solar incident angle (i), the angle between the cur-rent position of the sun (depending on solar zenith angle

and solar azimuth) and the local surface (terrain slope and

aspect) (see Fig. 2).

Figure 2: Angles necessary for computing the incident angle

(based on Teillet et al. 1982).


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raum Petra Freder

The illumination (cos i) can be computed as follows:

angleaspect

angleazimuthsolar

anglezenithsolar

angleslope

angleincidentsolarlocal

where

)'(cossinsincoscoscos

a'

a

z

e

i

aazezei

The value of cos i varies from -1 to +1, whereas a value

topographic correction of satellite images for improved lulc classification in alpine areas

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