OR/16/006 Increased exploitation and additional opportunities

Colm J Jordan, Tom Dijkstra and Stephen Grebby. 2016. Risk information services for Disaster Risk Management (DRM) in the Caribbean: mainstreaming opportunities. British Geological Survey Internal Report, OR/16/006.

The Service Utility Document (Grebby et al, 2015) outlined the assessment of the services/products supplied via the eoworld2 initiative. One of the primary outcomes from that document is a review of the areas where the stakeholders/users believed that the EO products could provide additional exploitation and opportunities. The EO products provided opportunities in terms of:

Improved information:

• “The new land use/land cover maps are more precise and accurate where the old one is general” – Grenada.
• “The map quality is an improvement to the previous one (higher resolution) . . . more classes of land cover identified” — St. Vincent and the Grenadines.
• “Generally seems to be good and properly represented . . . there is no new information, but it shows an updated land cover map” – St. Lucia.
• “The landslide inventory map for Saint Lucia is a very important dataset for us, and it will certainly contribute a lot to improving the quality of the landslide map for the island” — CHARIM project team.
• “The 20 m elevation model is better than we currently have, which is a 30 m. The maps are useful, especially the 1 m, this is really what we need” — Belize.

Timely delivery of results and updates

Over and above the additional information that could be provided with inputs from EO data, the stakeholders/users also identified that the integration of EO data could enable them to undertake new work that was not previously possible based on current practices and information:

• “The land cover maps for Grenada, Saint Lucia and Saint Vincent are very important datasets for us, and they will certainly contribute a lot to improving the quality of the landslide and flood maps for these islands” — CHARIM project team.
• “This will result in on time delivery of maps which can assist in decision making and planning. When a map is done using other methods it sometimes takes too long to materialise and sometimes the land cover may have changed before even producing a map” — St. Lucia.
• “Can help with development decision making and help in preserving of watersheds” — Grenada.
• “The map can be useful in monitoring changes in forest cover…gives a good idea as to where development is taking place” — St. Vincent and the Grenadines.
• “I believe that the land cover maps generated through satellite takes so little time as doing it on site. This will result in on-time delivery of maps which can assist in decision making and planning” — St. Lucia.
• “Images on a 2-year interval from 2000 to future years would be appreciated” — St. Lucia.”
• “We have been asking for LiDAR, but for some projects these maps that you produce can substitute for the accuracy needed” – Belize.

The existing land use/land cover maps for the countries were produced from freely- available Landsat satellite imagery acquired ca. 2000. Although free, the data have only a moderate spatial resolution of 30 m. In contrast, the new land cover maps were predominantly generated from very-high resolution Pleaides satellite imagery, which provides an order of magnitude increase in the spatial resolution (2 m) of the resulting maps. Access to the Pleiades satellite imagery was provided through the ESA TPM scheme, but commercially such imagery typically costs upwards of €17 per km². Taking into account the overwhelmingly positive feedback from users, it is apparent that the enhanced information content, quality and subsequent benefits of the new maps do fully justify the costs. Moreover, land use/land cover mapping using EO data is generally more time and cost efficient than conventional field-based mapping. This was recognised by one local user:

• “EO products will enable me to reduce the overall expenditure of data collection” — Grenada.

The national-scale DEMs for St. Lucia and Grenada were generated using ASTER stereo satellite imagery with a spatial resolution of 30 m and vertical accuracies (RMS) better than 5 m. Although elevation data with a higher spatial resolution and accuracy can be acquired through traditional ground-based GPS surveys, this approach is extremely inefficient when covering large areas. Airborne LiDAR can provide accurate and high resolution elevation data, although surveys have to be commissioned and can therefore be very costly. At a cost of approximately $60US for a scene covering 3600 km², ASTER satellite imagery proves a cost effective means of generating a DEM for applications such as national-scale landslide and flood hazard mapping. If temporal coverage of the DEM is not an issue, then the 30 m ASTER Global DEM, generated using imagery acquired in 2000–2011, is available free of charge. As was initially planned for this project, optical stereo imagery acquired by the Pleiades satellites can be used to generate DEMs with a much higher spatial resolution (ca. 1 m) and vertical accuracy. Such imagery can be acquired for a cost in the region of €29 per km², and would enable the landslide and flood risk to be mapped more accurately.

Although offering a slightly enhanced spatial resolution and vertical accuracy over the 30 m ASTER-derived DEM for Belize, the 20 m DEM generated from SPOT satellite imagery is more expensive at a cost in the region of €2 per km². Accordingly, in the absence of a DEM produced through traditional means, the use of stereo satellite imagery is the most cost effective method of producing a DEM of Belize for national-scale flood risk mapping purposes.

The precise 1 m DEM for 100 km² of Belize provides a demonstration of the full potential in using optical satellite to generate accurate and high-resolution DEMs. In this case, the DEM was generated using state-of-the-art Pleiades tri-stereo satellite imagery. Again, the imagery was acquired free of charge through the ESA TPM scheme. Commercially, this type of imagery costs in the region of €50 per km². Whilst this is quite costly, the feedback from the users suggested that a DEM of this quality and accuracy is urgently required for accurate flood risk mapping in Belize because it far exceeds that of the existing DEM. Based on this alone, the cost associated with this Service appears fully justified with respect to the potential benefits and opportunities it can bring. The 1 m DEM delivered by the Service is comparable to the resolution and accuracy achievable using airborne LiDAR. In generally, the generation of DEMs from tri-stereo optical imagery is probably more cost effective than airborne LiDAR for mapping areas up to 200 km².

The ability to generate DEMs from the optical imagery was perceived to be one of the major strength of the Services. This was particularly true for the precise 1 m DEM for Belize, with one local user adamantly stating that data of that quality was what they urgently required to enable accurate flood risk mapping.

A major limitation of the EO Services was the dependency on cloud free imagery. Many users commented on the incompleteness of the rivers and roads layers owing to clouds and associated shadows in the imagery. Cloud cover is arguably one of the main restrictions on the use of EO data in tropical environments such as the Caribbean region, largely because of the Atlantic hurricane season. The probability of acquiring useable cloud-free imagery can be maximised by increasing the time duration of the acquisition window or to avoid tasking during known climatic events.

Although information on the validation of the EO products using conventional procedures was provided, it is apparent that some local users preferred the opportunity to undertake their own field validation. This may be partly due to the relative unfamiliarity of these users with EO-based products and conventional validation procedures. Although not possible in this project, more interaction with the local users prior to and during the assessment phase would have helped to improve the level of confidence these user have in the reliability of the EO products. Despite this, all users were able to recognise the substantial benefits provided by the use of EO data in their activities.