Digital Elevation Models - Basin Wide

1. Status

Digital elevation models derived for both basins SASS and SAI are now available.

2. Product Specifications

Size:	SASS around 1 000 000 km² - Longitude 3°00’ West – 16°30’ East - Latitude 26°00’ North - 35°00’ North SAI around 525 000 km² - Longitude 0°03’ – 9°49’ East - Latitude 10°58’ – 19°31’ North
Scale/Resolution:	90m
Georeference:	SASS: Projection: UTM Zone 31, Ellipsoid: Clarke 1880 (IGN), Datum: WGS84 SAI: Projection: UTM Zone 31, Ellipsoid: WGS84; Datum WGS84
File characteristics:	Format: GeoTiff Data type: Integer 16bit
Minimum Accuracy	DTED level 1 (90 m horizontal, Altitude precision: 6m relative; 16m absolute)
Benefits/Comments:	see explanations below
EO Data:	SRTM C-band
For more specifications refer to:	R2_1: Technical Specification and Service Cases Description

3. Illustrations

The DEM are shown here in graphic form below, elevation information being color coded.

SASS
SASS: Basin wide DEM

SAI
SAI: Basin wide DEM
AOI-3 - Menaka - Banibangou: Watershed delineation

4. Results

	Type/Size	Date	Readme
Hydrological Addon for Area of Interest 3: Mali, Niger	ZIP/2356 kB	14.02.2007	Readme
DEM - SASS: Algeria, Libya, Tunisia	ZIP/207 MB	31.01.2006	Readme
DEM - SAI: Mali, Niger, Nigeria	ZIP/85 MB	25.10.2006	Readme

5. Maps

	Type/Size	Date
2 Maps - DEM basin wide - SAI & SASS	ZIP/2203 kB	26.06.2007

6. Poster

	Type/Size	Date
Digital Elevation Models	PDF/6650 kB	26.04.2007

7. Explanation

extern SRTM - the Shuttle Radar Topography Mission - was a joint project of US NIMA, NASA, ASI and DLR. The objective of SRTM mission was to produce digital topographic data for 80% of the Earth`s land surface (all area between 60 norht and 56 south latitude), with data points located every 1-arc second (approx. 30 meters on a latitutde/longitude grid.

Two independent radar systems were operated simultaneously during the SRTM mission: the German X-SAR instrument (3,1 cm wavelength) and the American SIR-C instrument (5,6 cm wavelength). The C-band radar scanned the earth's surface with a 225 kilometre-wide swath and was able to completely map the covered areas, the x-band swatch - located within the C-band swatch was only 50 km wide.

Therefore:

SRTM C-band data DTED level 1 with a raster cellsize of around 90m is now available, whereas access to C-band data DTED level 2 with 30m cell size is restricted.
X-band data with 30m resolution are more easily accessible but are only available in 50 km wide geographic stripes and the cost/benefit ratio is not attractive.

The benefits of the DEM are:

SASS:

Input to SASS ground water model, allowing for improved accuracy and enables improved regional modelling
Helps in determining where drainage waters from oases are flowing

SAI:

Determination of flow accumulation (local depressions) – which control ground water recharge
Delineation of water sheds - helps in modelling inflow to mares
Use together with GW models: national - regional GW models operated by the users organisation now or in the future
Use for verification of the mapped mares. Mares should match with depressions, depressions without mapped mares are zones of fast infiltration

For more details refer to: R2_4: Design Justification and R3_4 Producer Handbook

8. User Assessment - Validation

April 2006: DRE - Niger
The geometric accuracy seems to be good, in comparison to the hydrographic network from ADS/DCW data base. SRTM DEM products are available for more for 2 years and are no longer very innovative. They are of great interest for global overwiew but are not suitable for the study of small areas that represent the main part of our job. Global assessment: medium.

January 2007: ANRH - Algeria
"Produit est jugé très intéressant, mais il doit être lissé pour éliminer les pixel isolés, sans quoi la production de produits dérivés est presque impossible."

March 2007: DRE - Niger
+ permettent d‘avoir une délimitation (théorique) des bassins et sous-bassins versants
+ facilitent l’identification des zones favorables à l’implantation de barrage et de retenues collinaires
+ facilitent la gestion des BV partagés par plusieurs pays
+ peuvent contribuer aux zonages agro-écologiques et à la detection du potentiel irrigable
+ facilitent le calcul des écoulements et le positionnement des ouvrages d’art
+ facilitent les opérations de CES/DRS et la détection des zones innondables

June 2007: DNH - Mali
ref: Bassin versants issus de SRTM - évaluation globale: satisfaisante
+ permettent d‘avoir une délimitation (théorique) des bassins et
+ sous-bassins versants peut permettre l’identification des zones
+ favorables à l’implantation de barrages et de retenues collinaires
+ c’est un outil qui pourrait éventuellement faciliter la gestion des BV
+ partagés par plusieurs pays peuvent contribuer aux zonages
+ agro-écologiques et à la détection du potentiel irrigable facilitent
+ le positionnement des ouvrages d’art facilitent la détection des zones
+ inondables en étant croisé avec d’autres données