Lidar Overview

Introduction

Lidar Digimap offers detailed Lidar data vertical aerial photography from the Environment Agency.

Uses of Lidar data are highly varied, from use in the creation of visual effects for virtual reality and film projects to archaeology, forestry management, flood and pollution modelling. Because it is so much more detailed than “traditional” digital terrain models, in some datasets down to 25cm resolution, there is a wealth of additional uses for the data. Looking at lines of sight for rural developments, mobile phone and wireless broadband signals amongst other things is easier and more accurate using Lidar data because of the inclusion of vegetation in the model.

DTM and DSM

Lidar data is a very accurate model of the earth’s surface. The data is captured by firing very rapid laser pulses (thousands of times per second) at the ground surface. By examining the laser energy reflected back from the ground the surface is captured as a dense cloud of 3D points.

These points are then converted in to highly detailed terrain models of the surface of the earth and by examining the reflections from both the ground surface and the vegetation canopy it is possible to derive both Digital Terrain Models of the bare earth surface (DTM) and also Digital Surface Models (DSM) that show the vegetation canopy.

Note the DTM and DSM data are available as composite datasets comprised of the most detailed up-to-date data available for each location. The composite data is suitable for studies of larger areas where change over time is less important. The data is also available to download as individual snapshots captured each year by the Environment Agency and is well suited to monitoring of change over time at a particular location.

Point Cloud

LIDAR point cloud is a collection of hundreds of millions, or sometimes billions of highly accurate 3-dimensional x,y,z points and component attributes. The gridded raster products such as digital surface and terrain models are derived from this point cloud.

Aerial Photography

Vertical

Vertical aerial photography is an airborne mapping technique, which uses a high-resolution camera mounted vertically underneath the aircraft to capture reflected light in the red, green, blue and for some datasets, near infra-red spectrum. Images of the ground are captured at resolutions between 10cm and 50cm, and ortho-rectified using simultaneous LIDAR and GPS to a high spatial accuracy. The Environment Agency has been capturing vertical aerial photography data regularly since 2006. Vertical aerial photography has been captured on a project by project basis each ranging in coverage from a few square kilometres to hundreds of square kilometres. Historic data are available for some areas where we have carried out repeat surveys. Photography can be captured in all survey conditions, including incident response where it is used to derive flood outlines. The photography is supplied as a raster dataset in ECW (enhanced compressed wavelet) format as either a true colour (RGB), near infra-red (NIR) or a 4-band (RGBN) dataset.

Oblique

Oblique aerial photography is an airborne mapping technique, which uses a professional grade DSLR camera to capture images out the left side of the aircraft. Images are geo-referenced using GPS systems to provide the position of the plane for each image. The Environment Agency has been capturing oblique aerial photography during incident response since 2010, and for bespoke surveys such as cliff line monitoring. Images can be captured in all survey conditions which can have a large influence on the quality of the imagery.

Multibeam Bathymetry

The Environment Agency’s Multibeam Bathymetry data archive includes digital elevation data derived from bathymetric surveys carried out by the Environment Agency for a range of applications and locations where surveys have been previously commissioned. Multibeam data is available at 50cm resolution and supplied in ASCII raster format which contains height, relative to Ordnance Survey Newlyn datum. This is a technical spatial dataset and requires specialist geographic information system software to open and manipulate the data.

CASI Multispectral Imagery

Compact Airborne Spectrographic Imager 1500 (CASI 1500) is a multispectral pushbroom system that acquires data in visible and near infrared (VNIR) light. Unlike camera systems that generally acquire data in three (red, green, blue) or four (red, green, blue and near infra-red) bands, the CASI 1500 splits light into several discrete bands, up to 288, although approximately 20 are more normally captured. It is capable of collecting data in discrete areas of the electromagnetic spectrum and can target key wavelengths that allow information about ground characteristics and ground cover type to be inferred. For example, the CASI 1500 can target the precise wavelengths of chlorophyll absorption or the algae fluorescence peak. CASI 1500 data have been acquired on a project basis, so there are not extensive datasets. These datasets have generally been acquired where the limited spectral information in aerial photography would not be sufficient to map a specific variable, such as a specific vegetation type or biomass.