DIGITAL ELEVATION MODELS-A Guidance Note on how Digital Elevation Models are created and used – includes key definitions, sample Terms of Reference and how best to plan a DEM-mission.

Digital Elevation Model

Executive Summary

The cost of creating DEMs can be significant, often running into millions of dollars, due to their diverse and extensive utility. The use of DEMs is abundant in spatial analyses. As such, a clear understanding of the range of DEM types and applications, their operational requirements, and pros and cons of each model is important before deciding whether to acquire existing data or commissioning a new survey. Having robust background knowledge can help plan the optimal DEM acquisition. 

This Guidance Note aims to compile such knowledge in one publication and thereby address all pertinent topics of DEM creation and use, including a workflow to facilitate the best way to plan a well-informed DEM-mission or proposal, primarily aimed at non-specialists. Given the technical nature and complexity of the subject, this Note points out specific sections and key tables to simplify the reader’s ease-of-use and navigation. Readers who are interested in the more technical aspects of DEMs should refer to publications such as Maune (ed.) (2007).

DEM Applications

This Note includes an extensive list of applications for a DEM, in topics such as water resources management, disaster risk management, geology, agriculture and several others (See Section I-2: DEMs Applications). When drafting a proposal for DEM creation, it is often useful to state other potential uses of DEMs in the proposal rather than only the intended one; in this way the potential return on investment (ROI) is clearer to the funder. The ROI’s maximum realization potential is achieved if the license of the generated DEM is made open.

The DEM’s usage greatly determines the DEM output’s technical specifications. For example, for the modeling of coastal erosion, the DEM must meet the optimal specification (spec) requirements for the coastal erosion modeling methodology. Several options may exist to produce a DEM that fulfills the required spec for the project. In these cases, it is necessary for the project bidder to describe in the technical proposal the detailed methodologies envisaged to create the output DEM.

DEMs can be created for terrains (land surface) as well as underwater (e.g. seabeds). Underwater DEMs are called Bathymetry, and their generation requires a different approach and use of instruments compared to terrestrial DEM. For underwater, acquisition methods are different depending on whether near-shore or off-shore bathymetry is required, the threshold typically being water depth of 50m where beyond that sonar equipment are used. The costs of bathymetry data acquisition and generation is generally speaking 4–5 times more than that of terrestrial DEMs.

Modalities of DEM Generation 

DEMs need to be extensive and exhaustive in spatial scope using remote sensing, to be truly useful. Remote sensing, from an airborne (e.g., aircraft or drone) or a spaceborne platform (e.g., satellites), represents one of the best approaches for the development of a large area, high-spatial-resolution DEMs. The diversity of remote sensing modalities used to generate DEM products presents a breadth of choices, each with their relative strengths and Digital Elevation Models – A Guidance Note on How Digital Elevation Models Are Created and Used weaknesses and different types of output. 

The objectives, scope, geographical location and budget of each project will determine which of the remote sensing approaches are most appropriate to the task. Section II-1: Operational Guide to Tender a Digital Elevation Model, discusses the different DEMs’ modalities, and Section II-2: Workflow to Acquire a DEM for Projects, describes five key steps to acquire a DEM for projects.

Three remote sensing technologies provide elevation data: Light Detection and Ranging (Lidar) is more automated and finely scaled; Radio Detection and Ranging (Radar) is more effective in foggy or cloudy conditions. Stereo photography (three-dimensional imaging), however, only collects ground elevation data for physically observed or imaged areas. 

Lidar offers dense 3D point clouds, vegetation-penetrating abilities, and multiple secondary applications. Radar also offers vegetation penetration, but lower spatial resolution and higher processing needs. This can result in lower quality and increased costs in some circumstances/situations. Stereo photography offers context through imagery but offers lower spatial resolution and only top-most surface heights. 

Spaceborne platforms offer accessibility and coverage, but at the cost of spatial resolution and horizontal and vertical accuracy, making them especially useful for large areas (e.g., regional-to-continental mapping). In contrast, airborne platforms have much higher accuracy but sacrifice coverage and accessibility in very remote areas.

Multiple variables also define the output quality and characteristics of a DEM. The most commonly quoted variables are the vertical accuracy and horizontal point spacing (resolution). For vertical accuracy, photogrammetric or Lidar systems are best for higher vertical resolution applications in the order of less than 1m. Medium or lower accuracy applications allow the use of Interferometric Synthetic Aperture Radar (IfSAR), in the order of 1m to 5m, and satellite archive data. 

Section II-3: Requirements and Options include Table 9: Key accuracy requirements for a range of application areas, which shows the required DEM vertical accuracy for various applications. Section II-3.3.vii: Budget Constraints, includes Table 10: DEM product costs for various remote sensing modalities and vendors, which shows some examples of DEM product types (some being commercial-off-the-shelf (COTS) products), their vertical accuracy, and the approximate price range and licensing conditions. It is worth mentioning that there are global COTS DEM products available at either no charge2 or at cost.3

However, in most instances, DEM applications in topics such as water resource management, disaster preparedness, or agriculture generally require a finer spatial resolution than what best global products can provide. The objectives, scope, geographical location and budget of each project will determine which of the remote sensing approaches are most appropriate to the task. Section II-3 and 4: Sustainability Matrix and Applications Requirements Matrix discusses the various modalities of DEM generation.

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