Thursday, 24 November 2016

LIDAR Data - Introduction


Topographical data measured by LIDAR (Light Detecting & Ranging) has been collected by the Environment Agency since 1998, and all of this data is now publicly available as Open-Data; since September 2015.  This data is mainly used to map coastal regions, flood plains and urban areas to assess for flood risk and has been collected by mapping the landscape using a laser, and although the majority of land that has been scanned is adjacent to coastal regions it also includes a number of inland regions, including uplands.

LIDAR data can be accessed using two forms; Tiled LIDAR data which includes all data gathered since 1998 and is available in a range of resolutions, and Composite LIDAR data which is derived from a combination of the full tiled dataset and has been merged and re-sampled to give a better spatial coverage.

Surveying Accuracy

The horizontal absolute spatial error in this LIDAR data is +/- 0.4m, while the absolute height error is less than +/- 0.15m; figures which demonstrate the high level of accuracy associated with LIDAR data.  Significantly, the reliability of LIDAR data outweighs that of the beige-coloured spot heights obtained by aerial survey on Ordnance Survey (OS) maps, which are quoted as having an accuracy of +/- 3m.  In fact, the accuracy is comparable (although not as accurate) to that achieved by common survey-grade GNSS receivers for height and via the level and staff method for drop.  It is also extremely good when compared to the use of a level and staff to pin point the position for both summit and bwlch, and as such, analysis of LIDAR could prove more accurate than GNSS-receiver surveys where the summit and bwlch are not accurately determined using the level and staff method. 

Listing Philosophy

The philosophy of listings authored by myself or for those co-authored with Aled Williams is to use the most accurate data that is available on any given day.  This includes altering the status of a hill if the latest data requires to do so, however marginal the result.  By doing so this negates the inevitable wait for a hill to be re-surveyed using a more accurate method, this waiting process has meant that some hills do not have their status correctly listed for a number of years after the initial survey, we deem this to be in no one’s interest, as once re-surveyed its status can always be changed again.  As the beige-coloured spot heights on OS maps are taken at face value in many listings, Aled and I have decided to favour LIDAR data over data listed by OS beige-coloured spot heights for the listings that we produce.

As our stated philosophy is to use the most accurate data available on any given day we are now using LIDAR data where no data exists for the hill either through surveying with GNSS receiver of through the use of a level and staff.  And as LIDAR data is quoted as having a vertical accuracy of +/- 0.15m we have decided to quote the height produced by using LIDAR data to one decimal place. 

A number of hill reclassifications will result through the use of LIDAR data and these will be detailed on the Mapping Mountains site in due course.

Aled Williams and Myrddyn Phillips (November 2016)

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