Setting the USGS Overage Flags in TerraScan

Introduction

For many years users have been working with the concept of ‘cutting overlap’ in LIDAR data production. With the onset of LAS v1.4 and the USGS LIDAR Base Specification, the way in which this concept is implemented has significantly changed.  Specifically, USGS standards require a change in the way overlap points are managed and stored. Terms arise such as: ‘Flags’, ‘bits’, ‘overlap’, ‘cut overlap’, or ‘overage’.  To help ensure that products generated using TerraScan meet USGS delivery standards, this article discusses the concept of ‘Overlap’ in the LIDAR industry, the USGS requirements for ‘Overage’ flagging, and how to QC the end-product to ensure compliance.

What is ‘Overlap’?

When two flight lines (or drive passes) appear next to each other, there is usually an area where their footprints overlap one another (Figure 1A). This definition of ‘overlap’ is commonsense, but is not quite what is meant in the context of this article. The action of ‘Cutting Overlap’ points is not the removal of all the points in the overlap area (Figure 1B), but it is the sifting of those points and determining which to keep and which to remove.  This usually means splitting the difference between the two flight lines and picking the points closest to nadir for respective flight lines (Figure 1C).  Taking this step in LIDAR production has a couple of advantages: It ensures a more consistent point density distribution throughout the dataset; and will also ensure the most accurate points within the overlap area are retained for further processing. Previous methods of working with these eliminated points was to delete them – now a big faux pau in LIDAR processing – or, move them to the Overlap Class (historically Class Code 12, but non-existent in LAS v1.4), and disregard them in future processing steps.

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