High Resolution Investigation using Landsat-5 and SPOT 4 Take 5

The objective of the research was to demonstrate the potential of ocean colour imagery for coastal water bodies by using 6-month time series of high resolution optical data (SPOT-4 at 20m resolution and RapidEye at 5m resolution) collected during the SPOT-4 TAKE 5 campaign.
Two coastal sites were studied where CNES acquired SPOT-4 images when the satellite was put into the Sentinel 2 orbit, and where complementary RapidEye and Landsat data are available: Chesapeake Bay and South Korea.

A literature review was made in order to understand and apply the methods used to extract water quality parameters from high resolution sensors like the ones on board of the Landsat satellites (TM, ETM, ETM+) or SPOT (XS).

Application examples

Comparison of TSM derived from the Nechad approach on MODIS and SPOT for Chesapeake Bay:

Several tests have been made:

Comparison of Landsat-5 with MERIS products (Kd490, Turbidity and TSM)

The algorithm study was done by performing an analysis on Landsat 5 TM data on the Chesapeake area. Landsat 5 is radiometrically closer to Sentinel 2 than SPOT 4.
Five different ratios were calculated: the ratio TM1 (0.45 - 0.52 µm)/TM3 (0.63 - 0.69 µm) for turbidity; the Secchi Depth ratio TM2 (0.52 - 0.60 µm)/TM3; the ratios TM4 (0.76 - 0.90 µm)/TM1 and TM3/TM1; according to studies reviewed in the literature.

Kd490 test

Here we used MODIS L2 OC imagery and compared them with SPOT 4 Take 5 directly using the best ratio, which was resulting from the previous analysis with Landsat 5- MERIS and adapting it to SPOT.

MODIS TSM and SPOT

Here the ratios were compared only around the Chesapeake Bay (TSM calculation made for Chesapeake exclusively by Ondrusek et al., 2012)

The Nechad approach

In the paper published by Nechad et al. (2010), a single band algorithm for TSM retrieval based on a reflectance model is developed and calibrated, using seaborne reflectance and TSM measurements in the North Sea area. This algorithm has a simple application to multiple ocean colour sensors, taking into account that it is a semi-empirical approach on a determined area and with its particular spatial and temporal variability on specific IOPs.
For an estimation of the TSM concentration using SPOT 4 Take 5, an adaptation of the coefficients to fit the band 2 (610-680nm) has been done.
S -TSM concentration- is estimated from the water leaving reflectance using the following formula:

Eq. 1

where , , and are calibration coefficients related to the absorption and backscattering properties of the water combined with a simplified reflectance model based on a first order version of the model of Gordon et al. (1988).

After integration of the parameters, the values for this band are: =281.5773 (gm-3), =2.0543 (gm-3) and =16.3975 x 10^-2 (dimensionless) respectively. Therefore, the formula used stays like:

Eq. 2