High resolution mapping of urban areas using SPOT-5 images and ancillary data

14 Pages·2015·1.4 MB·English

International Journal of Environment and Geoinformatics 2(2), 63-76 (2015)
High resolution mapping of urban areas using SPOT-5 images and
ancillary data
Elif Sertel1*, Semih Sami Akay2
1Istanbul Technical University, Department of Geomatics Engineering, 34669, ISTANBUL-TR
2Istanbul Technical University, Center for Satellite Communication and Remote Sensing, 34669, ISTANBUL-TR
Corresponding author* Tel: [phone] Received 01 Feb 2015
E-mail: [e-mail] Accepted 01 Sept 2015
Abstract
This research aims to propose new rule sets to be used for object based classification of SPOT-5
images to accurately create detailed urban land cover/use maps. In addition to SPOT-5 satellite
images, Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index
(NDWI) maps, cadastral maps, Openstreet maps, road maps and Land Cover maps, were also
integrated into classification to increase the accuracy of resulting maps. Gaziantep city, one of the
highly populated cities of Turkey with different landscape patterns was selected as the study area.
Different rule sets involving spectral, spatial and geometric characteristics were developed to be
used for object based classification of 2.5 m resolution Spot-5 satellite images to automatically
create urban map of the region. Twenty different land cover/use classes obtained from European
Urban Atlas project were applied and an automatic classification approach was suggested for high
resolution urban map creation and updating. Integration of different types of data into the
classification decision tree increased the performance and accuracy of the suggested approach. The
accuracy assessment results illustrated that with the usage of newly proposed rule set algorithms in
object-based classification, urban areas represented with seventeen different sub-classes could be
mapped with 94 % or higher overall accuracy.
Keywords: Urban mapping, remote sensing, SPOT-5, object-based classification
Introduction Identification and quantification of urban areas
and having information about the rate and
The majority part of the world population has trends of urban growth provide beneficial
been living in urban areas and this number has inputs for urban and regional planners (Thapa
been steadily increasing. Similar to several and Murayama, 2009). Urban landscapes are
cities in the world, majority of the cities in variable in space and time; therefore, high
Turkey have been experiencing controlled or resolution multi-temporal satellite images are
uncontrolled expansion of urban areas as a important data source for spatiotemporal
result of development of transportation analyses of urban landscapes. There have been
infrastructures, industrial areas, settlements and several efforts on deriving urban information
other built-up areas (Akay, 2014; Seifert, and monitoring urban areas using remotely
2009). Rural migration to big cities and sensed data and details of remote sensing based
population growth are the main reason for urban monitoring for Europe could be found in
urbanization. Urbanization has significant Seifert, 2009. Comprehensive knowledge about
impact on the environment like causing landscape characteristics of urban areas is
modification of landscape, air pollution, crucial for the future development and effective
temperature increase etc. management of these areas (Deng et al., 2009).
Remote sensing is one of the fastest and most
preferred methods of producing information
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about the land use/cover of cities and countries. regional planning applications such as mapping
High resolution satellite images are extremely and monitoring urban changes, urban growth
useful to be used for spatiotemporal monitoring and land use dynamics, urban infrastructure
of cities, planning and development plans and characterization, urban road mapping, urban
decision support for city managers and functions analysis, environmental and resources
planners. Fast, accurate, cost and time effective management, infrastructure and transportation
map production and urban planning projects planning and development (Seifert, 2009;
could be successfully employed using remote Ludlow and Steinborn, 2009; Weng, 2012).
sensing technologies (Seifert, 2009; Deng et al.,
2009; Blaschke et al., 2011; Sertel et al., 2008). Considering the complexity of the urban
landscape due to high spatial and spectral
The European Urban Atlas is one of the latest variations in urban surface materials, accurate
initiatives in Europe, a land monitoring service mapping of urban areas is a challenging task
as a component of GMES (Global Monitoring (Blaschke et al., 2011; Herold et al., 2003;
for Environment and Security) / Copernicus Weng, 2012; Mohapatra, and Wu, 2011; Lu,
program, to create reliable high resolution land and Weng, 2006). Impervious land cover have
use maps of Large Urban Zones (LUZ, having similar spectral properties especially when only
population of 100,000 or more) to monitor and limited number of bands were used; therefore,
manage urban areas. Main data source for the inclusion of spatial and texture information into
land use map production is high resolution urban area classification is necessary (Salehi;
satellite images (2.5 m or better) in addition to 2012) Different researchers have applied
several ancillary data like COTS (Commercial various approaches to improve urban land
off-the-shelf), FTS (Fast Track Service), use/cover accuracy with the inclusion of
topographic map etc. Urban Atlas includes 22 geographic data, textural features, contextual
urban classes (artificial surfaces) in four information, expert systems and fuzzy
hierarchical levels and four non-urban classes. classification (Thapa and Murayama, 2009;
The scale of Urban Atlas is 1:10,000 and the Deng et al., 2009; Weng, 2012; Mohapatra, and
minimum mapping unit (MMU) is 0.25 ha for Wu, 2011; Lu, and Weng, 2006; City
the artificial surfaces but in some exceptional Development Information. , 2014; Gamba et al.,
cases this could be 500 m² and MMU is 1 ha for 2005; Matikainen and Karila, 2011). The
the other surfaces (EEA(European Environment importance of different requirements for spatial,
Agency)) (Urban Atlas Final Report, 2009). spectral and temporal resolution and geometric
Urban Atlas database is aiming to provide characteristics of urban features to map
homogeneous and up-to-date spatial impervious surfaces in the urban areas using
information on urban environments, allowing remotely sensed data was emphasized (Herold
intercomparison of different European cities et al., 2003). However, most of these researches
(Ludlow and Steinborn, 2009). conducted with high resolution satellite images
were either applied to a few number of urban
Scale, application area, source data and types of classes or to a small region compared to real
users should be considered during the selection size of a city.
of classification scheme for land use/cover
mapping. In this research, we used This research aims to create an accurate and up-
classification scheme of Urban Atlas project to-date urban land use/cover map of Gaziantep
which is mutually exclusive, exhaustive and city (total area of 6,558 km²), Turkey by
hierarchical and including classes like urban developing rule based decision trees including
areas with different densities, industrial, spectral, geometric and spatial characteristics to
commercial, public, military and private units, be used for object-oriented classification of
roads, railways, ports etc. These classes will be high resolution satellite images with the
very functional and supportive for different integration of different thematic layers to
level of users from decision makers and automate urban mapping procedure. Twenty-
planners to public. In addition, urban land two different land cover/use classes (19 of them
cover/use classes applied in this research could related to urban and artificial surfaces) were
be used for various mapping, urban and produced by applying the proposed approach
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with an overall accuracy of 95.46% and kappa region (Figure 1). Gaziantep is one of the oldest
value of 0.932. cities in the world having continuous
inhabitation. Gaziantep is an important
Study Area economic center of Turkey with several large
industrial businesses and the largest organized
Gaziantep is one of the world's oldest industrial area of the country. Industrial and
residential areas and it is center of culture and economic developments have resulted in
commerce with historic Silk Road. Being an expansion in residential and industrial areas of
important gateway, Gaziantep’s location is the city (City Population; 2014).
within the southeast part of the Mediterranean
Fig. 1 Location of the study area (a) Turkey’s location (b) Gaziantep (c) 7 regions of Gaziantep
Gaziantep, with its investments in varied 8th largest city in Turkey. The population of
sectors like industry, commerce and tourism, Gaziantep has experienced an annual growth of
has risen to the forefront of Turkey’s 50,000 per year, with the numbers counting of
developing cities. The population of the city of 1,700,763 in 2010; 1,753,596 in 2011 and
Gaziantep has been increasing as a result 1,799,558 in 2012 (Akay, 2014). Parallel to the
development of the regional economy due to rising population, the rapid spread of residential
investments, and influence of immigration. and industrial areas has been observed.
With a population of over 1,000,000, it is the
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Data third level whereas discontinuous urban fabric
divided into four classes (codes 11210-11240)
Six SPOT-5 images obtained in 1, 6 and 8 of based on the soil sealing value in the fourth
August 2013 were used as primary earth level. Road and rail network and associated
observation data for this research and treated as land class (code 12200) in the third level is
primary data source for the delineation of divided into three sub-classes in the fourth level
different land cover/use classes. Land cover/use namely fast transit roads and associated lands
classes used in this research are adapted from (code 12210), other roads and associated lands
Urban Atlas project nomenclature that is given (code 12220) and railways and associated lands
in the Figure 2. There are four different (code 12230). Detailed information regarding to
hierarchical levels in the classification scheme. explanation of each class could be found in
Urban fabric is divided into continuous (code Myint et al., 2011 and Seifert, 2009)
11100) and discontinuous (code 11200) in the
Fig. 2 Land use/cover classes used in the study
Multi-temporal Landsat-8 images obtained on also used. Point of interests like schools,
18th of June, 20th of July and 21st of August cemeteries, public buildings are available in
2013 were used to create Normalized this map as attribute; therefore, this information
Difference Vegetation Index (NDVI) maps. was used for the determination of some fourth
This multi-temporal NDVI data set was used to level urban classes used in this study like
determine general boundaries of forest and industrial, commercial, public, military and
vegetated areas visually and to collect training private units.
sites.
Forest type maps, Open Street Map road data,
In addition to satellite images, 1:1,000 scale CORINE map, online maps that provides visual
master zoning maps of central locations were information and features about locational
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objects (Google Earth, Yandex, Bingmaps etc.) which was also inferred from Urban Atlas
were also used in this study. Forest Type maps project. After the orthorectification of each
and online maps were used to aid interpretation SPOT-5 images, all of the six images were
procedure for training site selection. Moreover, mosaicked to create a complete image-map of
these online maps were also used to find out Gaziantep city.
land use information of industrial, residential
areas and road types. Classification
High (>1m and <2.5m) and very high
Soil Sealing Layer based on Fast Track Service resolution (≤1m) (HR and VHR) satellite
specifications was created from SPOT 5 NDVI images such as, SPOT-5, SPOT-6, IKONOS,
imagery for the determination of fourth level Worldview, Pleiades could be used for the
discontinuous urban classes based on different classification of urban land use/cover classes.
soil sealing values (code 11210-11240). Introducing HR and VHR satellite images into
urban mapping lead higher levels of detailed
All of the mentioned data sets were also used features; however, having higher spatial details
for the accuracy assessment after conducting exhibit complicated spectral characteristics due
classification. to the fact that very small objects could be
mapped. Traditional pixel-based image
Methods classification methods rely on spectral
characteristics and do not count for spatial,
Geometric Correction topological and textural features; therefore,
Geometric accuracy and scale of land use maps these methods fail to create highly accurate
to be used for urban studies are out most urban land cover/use maps from HR and VHR
important, therefore spatial resolution selection images since urban classes in these images pose
and geometric correction procedures should be complex spectral characteristics. These
carefully conducted prior to further analysis of problems could be overcome using object-
satellite images. As an example, satellite based classification techniques which do not
images having spatial resolution of 2.5m or only deal with spectral information but also
better, with 5m positional accuracy are required spatial, textural and hierarchical features after
to be used in European Urban Atlas project segmentation of imagery (Salehi et al., 2012;
which has the scale of 1:10,000. Walker, and T. Blaschke, 2008; Campbell,
2007; eCognition, eCognition Developer, 2010;
In our research, SPOT-5 images with 2.5m Lobo, 1997).
resolution were orthorectified to remove
geometric distortions of images and make them Object-based classification approach includes
compatible to be used with other geographic two step namely segmentation and
data sets and conduct absolute measurements. classification. In order to obtain accurate results
Orthorectified image of city center were and conduct fast classification, Gaziantep city
compared with 1:1000 scale maps to check was divided into seven sub-regions (Figure
their locational accuracy. Coordinates of 20 1(c)). These seven sub-region areas include one
control points were read from orthorectified city center region and six rural regions, and
imagery and these values were compared with each of them classified separately. In the
the coordinates obtained from 1:1000 scale map segmentation stage, image pixels were grouped
with higher positional accuracy, to find out the into segments based on shape, color, texture
final geometric accuracy of orthorectified characteristics and spatial relationships. Scale
SPOT-5 images. Comparison analysis parameter is used for the creation of different
illustrated that RMSE of control points were size objects. Bigger objects could be obtained
generally between 1 and 3.5 m and extreme using bigger scale parameters and small objects
maximum RMSE of 4 m was obtained only for could be obtained using smaller scale
few control points. It was concluded that parameters for a constant spatial resolution.
locational accuracy of SPOT-5 images used in Color parameter is affected by the spectral
this research have 5m or better accuracy and characteristics of the objects.
compatible to create high resolution urban map
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In the classification stage, objects in the images city center of the city (Region 7). For that
are assigned to land use/cover classes with the reason, segmentation/classification process of
spectral reflectance of the earth objects, this region differs from other 6 regions. It
patterns, depending on objects’ location and ensured good delineation of land parcels and
other features’ parameters. In addition, thematic roads. Furthermore, it was also used to assign
layers can also be used as ancillary data that all artificial surface classes. Urban sub-classes
allows the assignment of objects in the were classified with FTS data. Corine data was
classification process. Table 1 shows thematic used to assign forest class by code number in
data used in segmentation and classification the thematic layer. OpenStreetMap road data
process with respect to regions and classes. was used in the segmentation process which
resulted better delineation of roads.
Master Zoning Maps (1:1,000) acquired from
Gaziantep Municipality was only available for
Table 1 Thematic Layers used in segmentation/classification process
Thematic Layer Region Purpose
Master Zoning Maps Region 7 (city Segmentation/Classification of Artificial Surfaces
center) (1xxx)
FTS (SPOT 5 NDVI) All regions Classification of 11000
Corine 2006 Regions 1 to 6 Classification of 30000
OpenStreetMap Road Regions 1 to 6 Segmentation
Data
Table 2 Segmentation parameters for sub-regions
Sub-region no Sub-region name Scale Shape Compactness
1 Nizip-Kargamis 65 0.7 0.5
2 Nurdagi-Islahiye 70 0.6 0.5
3 Oguzeli 60 0.7 0.5
4 City center-west 65 0.8 0.5
5 City center-east 70 0.6 0.5
6 Yavuzeli-Araban 60 0.7 0.5
7 City center 75 0.7 0.5
Different segmentation parameters were applied indices used in classification process. Figure 4
to seven sub-regions of the research area to find summarizes how these indices and functions
out the best combinations for the segment used in the object based classification.
identification of the related sub-region. Scale,
shape and compactness of different sub-regions For 6 rural regions of Gaziantep city,
were presented in Table 2. Normalized Difference Water Index (NDWI)
was used to identify water and non-water areas.
Figure 3 shows an example of segmentation Different thresholds were applied to find out an
results for Region 7 (city center) which appropriate threshold value to identify water
conducted by incorporating master zoning maps areas (code 50000) using SPOT-5 data. Based
into segmentation procedure resulting in on analysis of different segments representing
detailed determination of roads. water class, NDWI value of 0.15 was found as
a threshold to identify water bodies. Therefore,
After the segmentation, several indices and segments having NDWI value of 0.15 or higher
functions were used to extract different land were assigned to water class.
use/cover classes. Table 3 explains features and
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Fig. 3 Thematic layer (master zoning map) integration into segmentation for Region 7 (a) segmentation without master zoning maps (b) segmentation
with master zoning maps
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Table 3 Indices and functions used in the research [adopted from Thomas et al., 2003]
Measures of Spectral Response
Standard deviation of The standard deviation of the NIR band derived from intensity values of
band 4 all pixels in this channel.
Mean value of band 4 Mean intensity values in the NIR band.
Brightness Mean of the brightness values in an image.
NDVI Normalized difference vegetation index; NDVI = (NIR — RED)/(NIR
+ RED)
NDWI Normalized difference water index; NDWI = (GREEN — NIR) /
(GREEN + NIR)
Measures of Shape
Shape index Smoothness of the border of the image object.
Border length The sum of the edges of an image segment.
Asymmetry Length of an image object, in comparison to a regular polygon.
Elliptic fit Measures the similarity of object’s shape to an ellipse.
Area The total number of pixels in the object
Measures of Relation
Relations To Determines the relation of objects with various parameters to a given
Neighbor Objects class.
Fig. 4 Indices and functions used in object-based classification.
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Multi-temporal Normalized Difference incorporated in the segmentation process which
Vegetation Index (NDVI) images created from then was used to assign artificial surface classes
Landsat images and forest type maps obtained such as 12100, 12230, 14100 etc.
from Ministry of Forestry were incorporated
into the classification procedure for the At the last step, a confusion matrix was created
selection of training sites to be used in object- using reference points distributed randomly to
based nearest neighbour classification to check the accuracy of the proposed
identify forest and semi natural areas (code methodology. Overall accuracy, user’s and
30000). For different forest types, different producer’s accuracies of each class were
NDVI ranges were determined based on the calculated and the results were analyzed to find
training sites. Railroads and highways in vector out the performance of current approach.
format obtained from Open Streetmaps were
integrated into the segmentation procedure to Results and Discussion
clearly identify road and rail network (code
12200) and its sub-classes. To disengage class Current urban land cover/use map (Urban
other roads and associated land (code 12220) Atlas) of Gaziantep city with the total area of
from other classes, “elliptic fit”, “border 6,558 km2 was created by using object based
length”, “shape index”, “brightness” and classification of high resolution SPOT-5 images
“asymmetry” functions were applied to SPOT-5 with the aid of other data sources. Different
images and road and rail networks were decision tree algorithms were developed in this
classified. Besides, to distinguish roads research to accurately map different urban
between the agricultural areas, NDVI and classes. Results of the study showed that most
brightness values were used. NDVI values, soil of the urban classes were classified with 90% or
sealing layer, standard deviation and brightness more user’s and producer’s accuracies.
values in NIR band were used to identify
artificial surfaces. Human influence is dominant SPOT-5 images itself are not solely enough to
on artificial surfaces except agricultural use. map urban classes used in this research
Built-up areas, roads, infrastructure considering that some of them are related to
constructions and other artificially sealed areas land use. However, integration of different
are example for artificial surface class.16 thematic layers into the classification procedure
Different threshold values were applied to soil significantly improved the classification results
sealing layer as illustrated in Urban Atlas and helped to produce several urban land cover
mapping guide to accurately map class 11100, and use classes.
11200 and sub-classes of 11200. To identify
continuous urban fabric (11100), objects in After conducting the classification, it was found
urban classes having 80% or bigger values for that water areas are occupied 1% of the city
soil sealing were selected. This class mostly with a total area of 60 km2. Percentage of urban
includes buildings, road and paved areas. As areas are 5% with a total area of 291 km2
can be seen in Figure 6 and 7, Gaziantep city including continuous urban fabric and three
center is mostly covered by this class. different discontinuous urban fabrics with
Discontinuous urban fabric (11200) was different densities. Urban density map could be
classified using urban class and soil sealing created based on these classes. Forest area
values between 10% and 80%. Sub-classes of covers 9% (616 km2) and agricultural area,
discontinuous urban fabric are 11210 with semi-natural areas and wetlands cover 5,591
sealing layer value of 50-80%, 11220 with km2 with the highest percentage of 85%.
sealing layer value of 30-50% and 11230 with Gaziantep urban land cover/use map is
sealing layer value of 10-30% and 11240 with presented in Figure 5. Total area of water,
sealing layer value of 0-10%. urban and forest area cover lower proportion
compared to agricultural, semi-natural and
After the classification of 6 regions with similar wetland areas.
rulesets, city center (Region 7) of Gaziantep
was classified with an additional dataset; master Land cover/use classes used in this research
zoning maps. Master zoning maps were adapted from Urban Atlas project and these
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classes could provide valuable information for Figure 6 illustrates the urban map of the city
different applications in city scale for different center. Mostly, urban sprawl could be seen in
levels of users from public to decision makers radial direction around the city center of
and city planners. For example, transportation Gaziantep and in some parts linear direction
classes (code 12210-12230) could be used for along the highways.
route mapping, shortest distance calculation and
emergency planning. Green urban areas class Classification results were converted into
(code 14100) can be used to find out extent and vector format and 634,671 polygons were
spatial distribution of green areas which serves created to represent different classes of
as a meeting place and support social contacts Gaziantep urban land use/map.
in daily life and this information could be used
for further planning these areas.
Fig. 5 Gaziantep urban land cover/use map
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