SLOPE, Curvature, Aspect
Slope map (Source: WesternGeco)
- Seismic Planning - Areas of poor coupling
- Seismic Planning - Identification of adverse terrain for trafficability
- Surface Geology Mapping - Mapping geological features
- Surface Geology Mapping - Lithological discrimination
- Surface Geology Mapping - Terrain evaluation and geo-morphology characterization
- Logistics Planning and Operations - Baseline mapping of terrain and infrastructure
Slope, curvature and aspect are derived from a DEM. The terms DEM, Digital Surface Model (DSM) and Digital Terrain Model (DTM) are often used interchangeably, the DSM will take into account the surface features (such as buildings or trees) and DTM will focus on bare earth. Most data providers use the term DEM as the generic term to describe DSM and DTM, as such DEM will be used for this product sheet.
Slope, aspect, and curvature are key variables that can be found by computing the gradient, or the change in the z-value (height), from continuous DEM raster. These data highlight structures on the earth’s surface.
Slope is a measure of the rate of maximum change in the z-value (height) in a DEM cell. Slope can be measured in degrees from 0-90, where 0 is flat or percentage where 45 degree is 100 percent.
Curvature is a second derivative of the surface, i.e. the slope of the slope. A curvature dataset is useful for environmental flood monitoring; it can give an approximation to the flow of water over a surface.
Aspect is a measure of slope direction; the values of an aspect dataset is normally defined in degree, ranging from 0 (north) to 90 (east) to 180 (south) to 270 (west).
Known restrictions / limitations
- High latitudes coverage is restricted for free SRTM data.
- When generating DEM from stereo pairs, good quality imagery needs to be available with 2 or more images showing the same area from different directions. This can be a time consuming process, if the imagery is in archive then 3 week turnaround time, running into months if the satellite needs tasking. Lower resolution DEM data is available off the shelf. Cloud cover and shadow are a significant limiting factor in creating a DEM from Stereo pairs.
- Radar derived DEMs are available off-the-shelf, with accuracy affected in steep mountainous regions and densely vegetated regions.
- In dense vegetation where bare earth models are needed (for example in Seismic Planning) EO derived products cannot provide a solution and as such LiDAR is the most commonly used dataset.
Lifecycle stage and demand
All lifecycle stages: Can be used at all stages with the cycle. Elevation mapping is useful for the oil and gas industry some examples below.
- Reducing vehicle rollover risk
Surface Geology modelling
- Understanding structure, near surface modelling, geomorphology, 3D mapping
Logistics planning and operations
- Health and safety issues, slope modelling, accessibility issues, drainage, site planning
Geographic coverage and demand
Global coverage (with a few restrictions see below).
OTM:058 Identifying ground conditions susceptible to poor coupling
OTM:045 Identifying soft ground for seismic vehicles
OTM:046 Identifying variations in trafficability for seismic vehicle
OTM:044 Identifying steep terrain for seismic vehicles
OTM:025 Early identification of potential hydrocarbon basins
OTM:051 Identification of fault lines
OTM:052 Identify the cause of geological movement
OTM:054 Understanding the near-surface for anticipating seismic signal absorption properties
OTM:055 Obtaining detailed terrain mapping for DEM construction
HC:1101 Identify areas with soft sediments to avoid strong attenuation
HC:1103 Identify soft and hard ground as areas of potentially poor source and receiver coupling
HC:1201 Identify up-to-date general land use patterns to plan access and apply safe setback distances.
HC:1202 Identify rivers, lakes and wet areas to apply safe setback distances
HC:1203 Identify areas with soft sediments to plan access and assess hazards
HC:1204 Assess forest characteristics to plan access and assess hazards
HC:1205 Identify steep slopes to assess potential constraints to access in forested areas
HC:1206 Identify steep slopes to assess potential constraints to access
HC:1207 Identify claypan surfaces to be avoided
HC:1211 Planning bridging through a tropical forest
HC:1303 Planning heliports, camps, and drop zones in forested areas
HC:2101 Lineament mapping
HC:2201 Identify geological structure through landform
HC:2401 Identify geohazards and landscape change rates
HC:2502 Identification of problem soils
HC:2503 Assessment of duricrusts and rock excavability
HC:2504 Identification of slope instability
HC:3101 Baseline and monitoring of areas with active faults and subsidence
HC:3203 Management of surface impacts due to ground deformation from operations
HC:3302 Assessing ground deformation to support enhanced recovery operations
HC:4103 Social baseline information to support compensation and/or resettlement
HC:4105 Identification of cultural heritage and archeology assessment
HC:4202 Map coastal habitat and built environment/settlement sensitivity to strengthen tactical oil spill response and preparedness>
HC:4206 Monitoring lake and wetland levels and recharge rates following water use for exploration/operations
HC:4207 Understanding and predicting changes in hydrological processes
HC:4301 Map and monitor induced seismic hazards
HC:4302 Floodplain mapping and understanding flood extent and flood frequency.
HC:4307 Coastal elevation data for tsunami risk analysis
HC:5104 Baseline elevation data for project planning and design
HC:5302 Terrain stability for route planning
Input data sources
Optical: VHR1, VHR2, HR1, HR2
Radar: VHR1, VHR2, HR1, HR2, MR1, MR2
Spatial resolution and coverage
Spatial resolution: 1 m – 1 km pixel size
Minimum Mapping Unit (MMU)
n/a (the product is directly based on the input data; the smallest unit is one pixel)
Accuracy / constraints
Spatial accuracy: For horizontal accuracy the goal would be one pixel, but depends on reference data
Accuracies for a few off-the-shelf elevation products:
- SRTM version 3: Absolute and relative vertical accuracy was anticipated to be less than 16 and 10 m, respectively
- ASTER GDEM2 have a root-mean-square error (RMSE) in elevation between ±7 and ±15 m can be achieved with ASTER stereo image data of good quality.
- WorldDEM (TanDEM-X) has a 2 m relative and a 4m absolute vertical accuracy in a 12 m x 12 m raster
- WorldView Elevation Suite (for a 1 m x 1 m DEM) 30 cm relative vertical accuracy and 50 cm relative horizontal. However the accuracy is dependent on the quality of ground control points (GCP). Known locations need to be identified in the images.
Accuracy assessment approach & quality control measures
Field survey spread over 16 locations of a 25 km2 area DEM, using DGPS or RTK surveying and processing the results.
Frequency / timeliness
Frequency: The frequency is constrained by satellite revisit and acquisition, but also processing requirements. Depending on the requirements of the customer the best suitable satellite sensor has to be chosen regarding spatial / spectral resolution as well as revisit frequency and timeliness. For coarser resolution DEMs, static products exist for most parts of the globe.
Timeliness of deliverable: As fixed models are used for production, the product can be generated within a day if the area does not exceed a certain area (premise: the DEM is already available).
1 km (GTOPO30), 90 m (SRTM version 3) and 30 m (Aster GDEM2) DEM are freely available. For higher resolution this data needs to be commercially acquired. These data sets for example are Elevation 30 (SPOT DEM 30 m), WorldDEM (TanDEM-X 12 m).
For higher resolutions (e.g. Elevation 8 (SPOT DEM 8 m), Elevation 4 & 1 (Pleiades DEM 4 m & 1 m), WorldView Elevation Suite (1 m)) processing will have to take place, whether there are suitable images in archive or if they need to be tasked. Working to time frames that fit in with GCP collect is important as well.
Delivery / output format
- Raster formats (depending on customer needs)
- Geotiff (standard - any other OGC standard file formats)