Impervious Surface Mapping

for Urban Storm Water Management

Introduction

Over the past centuries urbanization and increasing commercial and residential development has substantially increased the proportion of impervious areas. Unlike areas with natural water percolation, impervious surface areas do not allow runoff to seep into the ground. The impervious surface area therefore has a great impact on storm water runoff – an impact which will be even further exacerbated in the future where more extreme rainfall events are expected due to ongoing climatic changes.

Thus decision-makers and practitioners will need more than ever to know where all the excess water goes in order to mitigate the potential threats from extreme weather events and promote sustainable urban planning.

Spatially explicit information about the amount, location and type of impervious surfaces is required in order to run spatially distributed hydrological models for evaluating how storm water runoff will be affected by future developments in climate as well as the urban area.

DHI-GRAS offers a highly automated impervious surface area solution which is designed to provide practitioners and decision makers with the ideal reference information needed for hydrological modelling and urban planning. By reducing the dependency on visual interpretation the method is cost-effective and consistent.

Key benefits

• To be used for urban storm water management and storm water tax calculation
• Mapping can be done based on client’s ortho-photographs to save on the total cost, or based on satellite images
• Data is ready for use in hydrological modelling software or GIS
• Automated solution ensures consistent data and fast delivery
• Other area features can be extracted in the same processing

About the method

Aerial photographs and satellite images are important sources of information for imperious surface mapping. Both provide an overview of the survey area at a specific point in time, and can be used to differentiate various types of impervious surface areas (roads, rooftops, parking lots etc.) from the pervious areas.

Digital image analysis and classification has traditionally been performed using either simple manual interpretation or automatic method operation at the pixel level where each pixel’s digital number is assessed individually. However, with the recent advent of object-oriented image analysis, pixels are aggregated in a first step (segmentation) into objects that are homogenous with regard to spatial or spectra characteristics. Spectral, textural, shape or contextual criteria can then be applied to interactively classify the objects into a pre-defined set of classes.

For Impervious Surface Mapping, object- based image analysis is advantageous, because this approach allows for the detection of landscape objects – objects that can often be directly linked to topographical units such as segments of buildings and roads.

The ability to incorporate GIS themes such as road layers, building database information etc. directly into the segmentation and classification routines is a further advantage of the object-oriented approach which helps to improve the accuracy of the impervious area map.

Depending on the requirements of the analysis it is also possible to map different surface types that may have different environmental profiles. For instance, certain roof types such as copper will have a larger impact on water quality and the need for treatment.



Price and conditions

The price of Impervious Surface Mapping varies and relates to factors such as size of the area, input data types and availability of ancillary data etc.

Mapping can be done based on existing aerial photographs or based on satellite images in Very High Resolution with a spatial resolution of 0.5 – 1 m. For overall river basin mapping it is also possible to perform the mapping at smaller scales using medium resolution satellite images.

Please contact DHI-GRAS for further information about the possibilities for Impervious Surface Mapping.

Availability

• An up-to-date map of impervious surfaces can be created
• Historic analyses and comparisons back in time are also possible to assess changes in impervious areas

For more information view the GRAS website www.gras.ku.dk or contact DHI.