Chapter E

Satellite Imagery: 
Lake Pontchartrain Basin and Gulf of Mexico

¹ NOAA Climatic Data Center, formerly of the USGS

[Note: The text to this chapter was extracted from Open-File Report 98-484, Satellite Imagery of the North-Central Gulf of Mexico (Stumpf, 1998) and the web site http://coastal.er.usgs.gov/pontchartrain/imagery/ and modified by the editors for this presentation. The author has left USGS for a new position at the NOAA Climatic Data Center (Satellite Data Services Division). For more detailed background, see references at the end of this chapter, especially Stumpf (1998).]

Introduction
Methods and Technical Information
Applications to the Bonnet Carré Spillway Opening
North-Central Gulf of Mexico Imagery
Lake Pontchartrain Basin Imagery
Data Sources and References

Introduction

The studies reported here refer to satellite imagery collected by the NOAA Advanced Very High Resolution Radiometer (AVHRR) on NOAA polar-orbiting weather satellites. AVHRR provides nearly daily coverage of a site at resolutions of approximately 1 km.

Two scales of imagery are provided: data covering the region of the Gulf of Mexico from the Florida Panhandle to Louisiana (Stumpf, 1998) and expanded images of the Lake Pontchartrain area (obtained from website referenced above). Links to the imagery are located at the bottom of this page.

The data sets include measurements of sea surface temperature (SST), water reflectance, and false-color infrared imagery. The water reflectance measurements have been converted to provisional estimates of suspended sediment concentration (seston) and diffuse attenuation coefficient (K) based on algorithms developed for the area.

Methods and Technical Information

Satellite systems. The imagery was produced from data collected by the AVHRR on the NOAA polar-orbiting environmental satellites. NOAA-14 was the primary satellite used for this set of images, with an overpass time of 1400 local standard time (LST). The data sets were acquired from the Louisiana State University Earth Scan Laboratory and the University of South Florida/Florida Department of Environmental Protection Joint Use Facility. The satellites carry several sensors for meteorological studies. More details on the AVHRR and the satellites can be found in Kidwell (1991).

The NOAA polar-orbiters are in a sun-synchronous orbit, meaning that they will pass over a point at about the same time each day. The satellite has a nominal elevation of about 830 km and takes about 100 minutes to complete an orbit. The nadir point shifts about 3 degrees of longitude each day. About every 9 days, an area of interest is at nadir, centered in the satellite view area. Because of the movement in nadir position, during any 9-day period, an area is within the scanned region for 6 days.

AVHRR is a multiple-band scanning radiometer. The instrument scans one line at a time, with the movement of the satellite producing an offset between lines, resulting in the development of an image. The instantaneous field of view (IFOV) is 1.4 milliradians, corresponding to a ground resolution of 1.1 km at the nadir point. Because of along-scan overlap of the pixels, the nadir pixel area is nominally 0.8 km by 1.1 km. The instrument scans 2,048 pixels per line to a maximum scan angle from vertical of 55 degrees, covering a distance on the ground of about 2,500 km. Because of the extreme angle at the ends of the scanlines, geometric and atmospheric distortion become substantial for areas imaged at the limits of the scanlines. Scenes that fell at the edges of the satellite viewing area were generally not used because of the poor image quality.

The AVHRR has five channels on the current satellites, two reflected light and three thermal-infrared. The two reflected light channels (580-680 nm and 720-1000 nm) are used to derive water reflectance images for this study. These channels are used for a variety of purposes not addressed here, including mapping of vegetation, snow, ice, and clouds. The thermal infrared images are used to derive the sea surface temperature imagery.

Baseline data. The World DataBank 2 was used for overlays to these images. This database includes coastlines, State boundaries, and major rivers. A land mask was derived from the database shoreline.

Images. The false-color infrared imagery provides a photographic view of the area, allowing the user to identify cloud patterns, land features, and other patterns such as glint off the water. These images correspond approximately to color-infrared film. Bright red areas on land represent a dense, vegetated canopy. White areas indicate cleared, bare fields or urban areas. In the water, river plumes may appear as turquoise, and glint tends to have a whitish appearance. All other images have a color table included. Purple corresponds to low values (low temperature or reflectance); red and pink correspond to the highest values. Identified clouds are masked in gray or pink, land is masked in black or brown, the coastline is in white, political boundaries are in yellow, and rivers are in turquoise.

Applications to the Bonnet Carré Spillway Opening

The Bonnet Carré Spillway was opened from March 17, 1997 to April 18, 1997 to allow excess water from the Mississippi River to flow into Lake Pontchartrain, thus lowering the river stage and decreasing the possibility of flooding. (Baird, 1998, in Lopez, this volume). To illustrate the effects on the lake, remote sensing data of the lake and surrounding gulf are shown here for March, April, and May of 1997, as well as for the same months during 1996 for comparison. 1995 and 1997 images showing Lakes Pontchartrain, Maurepas, and Borgne are also included to allow closer inspection of the changes. Presented in this second set are algal bloom images from June and July of 1997, as well.

A plankton bloom built up throughout May, reaching a peak in mid-June (Dortch, 1998, in Lopez, this volume). Inorganic suspended matter tends to create high reflectivity, whereas plankton blooms have lower reflectance and may even be absorptive. The high-reflectance patterns observed from late March through April 18 may reflect particulate loadings from spillway discharge, modified by wind-driven current patterns (Signell and others, this volume). In contrast, the period after April 18 shows variable reflectance. This is particularly true for late May, when the blooms reached their greatest extent. More detailed comparisons between surface water physical, chemical, and biological observations may be required to resolve the significance of the patterns.

North-Central Gulf of Mexico Imagery

Lake Pontchartrain Basin Imagery

  Data Sources and References