The Validity of Open-Source Elevations for Different Topographic Map Scales and Geomatics Applications in Egypt and Saudi Arabia

The representation of the Earth’ topography in a 3D digital format is a vital constituent in a wide range of geographic, geomatics, environmental, and engineering activities. However, the accuracy of such representations could define its applicability and validity for a specific utilization. This chapter presents an analysis of four open-source Global Digital Elevation Models (GDEMs) and compares them on two topographic profiles (nearly flat, and hills regions) for mapping and geomatics applications in two countries. The main goal is to determine whether GDEMs-based heights, contour intervals, slopes, and topographic profiles are accurate for all topographic map scales, which is a significant problem in mapping operations. For the purpose of illustrating flat and moderate-topography patterns, two case studies—the Nile delta in Egypt and Makkah city in Saudi Arabia—have been used. The investigated GDEMs include the most-recent released models: ASTER v.3, ACE 2, SRTMGL1 v. 3, and NASADEM_HGT v.1 released in 2019 and 2020 with spatial resolutions of 1 and 3 arc seconds. There are 540 accurate Ground Control Points (GCP) available in the Nile delta and 175 in Makkah.

It has been determined based on the available datasets in two study locations that the accuracy of investigated GDEMs over known checkpoints range from ±2.5 and ±5.1 meters in the Nile delta region, while it varies between ±5.1 and ±8.0 meters in the Makkah area. This suggests that the use of GDEMs in topographic mapping varies greatly between spatial areas that are flat and those that are hilly. Therefore, it is advised against utilizing GDEMs to create topographic maps at scales of 1: 25,000 or greater in flat areas and 1: 50,000 or greater in hilly areas. Additionally, the achieved results demonstrated that throughout cross sections up to 30 kilometers in length, no GDEM-based slope matches the real slopes from known GCP. This leads to the conclusion that GDEMs are not the appropriate heights' source for topographic mapping at medium and large map scales, and could not be utilized for topographic profiling in precise engineering and geomatics applications. Such findings should be vitally taken into account in mapping and engineering applications not just in Egypt and Saudi Arabia but worldwide as well.