Satellite data

4.1.1 Landsat-7 ETM+

Landsat-7 is the most recent in the series of Landsat satellites, which have been used to acquire medium resolution imagery of the Earth since 1972 (USGS 01.03.2005). Landsat-7, carrying the Enhanced Thematic Mapper Plus (ETM+) instrument, was launched in April 1999. Four years later, on 31 May 2003, the scan line corrector failed, introducing data gaps in the ETM+ images and thus reducing the coverage of full quality Landsat ETM+ data.

The satellite has a sun-synchronous orbit with a 98-degree inclination and an altitude of 705 km. It has a swath width of 185 km and a repeat coverage interval of 16 days. The ETM+ is the latest version of the Thematic Mapper (TM) instrument carried by Landsat-4 and -5. The ETM+

instrument records essentially the same spectral bands as its predecessor, but it has an additional panchromatic band with 15 m ground resolution and a better ground resolution in band 6 (thermal infra-red) (table 2).

Table 2: ETM+ characteristics.

Band Number Spectral Range (µm) Ground Resolution (m)

1 0.45 to 0.515 30

2 0.525 to 0.605 30

3 0.63 to 0.69 30

4 0.75 to 0.90 30

5 1.55 to 1.75 30

6 10.4 to 12.5 60

7 2.09 to 2.35 30

Pan (8) 0.52 to 0.90 15

The Landsat-7 ETM+ scene used here has the path/row numbers 08/47 and covers the central and western Dominican Republic. It was acquired on 15 September 2000 at 14:59 GMT (Greenwich mean time). The illumination is characterized by a sun azimuth angle of 120° and a sun elevation

angle of 60.5°, and the image was processed to Level 0R (see metadata in appendix 3). The Level 0R Landsat product is reformatted raw data delivered with a radiometric resolution of 8 bit. Values are Digital Numbers (DN). The scene is very clear over the study area with no visible atmospheric effects except for a few small clouds (see figure 8).

Figure 8: Landsat-7 ETM+ image subset of the upper catchment area of the Río Yaque del Norte, RGB 583.

4.1.2 IKONOS

The IKONOS-2 satellite was launched in September 1999 and has been delivering commercial data since early 2000. IKONOS is the first of a new generation of high spatial resolution satellites which by now also include QuickBird-2, OrbView-3 and Eros A1. The IKONOS sensor uses a large telescope and pushbroom detector technology to simultaneously record four channels of multispectral data at 4 metre resolution and one panchromatic channel with 1 metre resolution

(nominal resolutions at < 26° off nadir). The spectral range of the multispectral channels is almost identical to that of the first four Landsat channels (see table 3). The IKONOS satellite is in a sun-synchronous orbit with a 14 days repeat cycle but is has a 1 to 3 day revisit time due to its pointing capability of 30° in any direction (Van der Meer et al. 2002). Further orbital information is listed in table 4. IKONOS data is collected as 11 bits per pixel (2048 grey tones).

Table 3: IKONOS-2 instrument characteristics.

Band Number Spectral Range (µm) Ground Resolution (m)

1 0.45 to 0.52 4

2 0.52 to 0.60 4

3 0.63 to 0.69 4

4 0.76 to 0.90 4

Pan 0.45 to 0.90 1

Table 4: IKONOS-2 orbital information (Dial et al. 2003).

Altitude 681 kilometers

Inclination 98.1 degrees

Orbit time 98 minutes

Orbit type sun-synchronous

Descending nodal crossing time ~10:30 a.m. local solar time Nominal swath width 13 km at nadir

Revisit frequency

3 days at 1 meter resolution (60° collection elevation / 26° off nadir) These values are for targets at 40 degrees latitude. The revisit times will be less frequent for latitudes closer to the equator.

Viewing angle Agile spacecraft - in-track and cross-track pointing

IKONOS is thus the first satellite delivering commercially available high resolution multispectral data with global coverage and constant acquisition parameters (apart from the variable viewing geometry). This makes it possible for the user to acquire near photographic quality satellite imagery of anywhere in the world (except for polar regions).

IKONOS data are marketed by Space Imaging. The data are collected on request only and can be ordered as a new collection (scheduling and tasking of the satellite), or from Space Imaging’s imagery archive if data of that location has already been collected. The pricing varies considerably depending on the area of interest (km²) and its location, the product (processing level) and the type of licence, among other factors. In Europe, the non-orthorectified ‘Geo product’ including all five bands cost 27.50 US $ per km² for a new collection and 21 US $ per km² for archived data in 2004.

The prices are higher for orthorectified products, stereo pairs and in case of ‘priority tasking’ (fixed acquisition time frame). The imagery which is delivered can contain up to 20 % cloud cover.

The IKONOS data which were available for this study belong to the ‘Geo product’ category. Image distortions which are due to the collection geometry are removed in this product, and the imagery is resampled to a uniform pixel spacing and a specified map projection. There is no correction of terrain effects, though (Space Imaging 2004).

12 IKONOS images of diverse quality (in terms of haziness and geometric distortions) cover the UCRYN. They are partly cloudy, so that they contain information about somewhat less than 100 % of the surface of the catchment area. They were acquired between September 2000 and June 2002.

Two of these images, which are of relatively high quality, were used for further processing (beyond visual enhancements). Their acquisition parameters are presented in table 5 (see also appendix 4).

Table 5: Acquisition parameters of IKONOS images used in this study.

Both images have relatively high sun elevations (68° and 75°), leading to only slight topographic illumination effects. The eastern image (figures 9 and 10) also has a high collection elevation (81°), while the Manabao image has a collection elevation of only 67°, leading to stronger geometric distortions through terrain displacement.

Eastern image (the eastern test area is a subset of this image)

Western image (Manabao).

Acquisition date/time 19 April 2001 15:19 GMT 22 May 2002 15:37 GMT Sun angle azimuth 109.9° According to metadata file 81.6°, but

looking at time of acquisition, sun angle elevation and shadows in the image, this must be an error; the real value is ~120°.

Sun angle elevation 68.3° 75.1°

Nominal collection azimuth 220.2° 322.0°

Nominal collection elevation 81.3° 66.9°

Acquired nominal GSD (ground sample distance) cross scan

0.83 m 0.91 m

Acquired nominal GSD (ground sample distance) along scan

0.83 m 0.93 m

Figure 9: IKONOS multispectral sub-image of the eastern test area (red dotted line: western border of the Scientific Reserve Ebano Verde), RGB 321.

Figure 10: IKONOS multispectral data, 4 m resolution, RGB 432 (left) and panchromatic data, 1m resolution (right).