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Using SPOT-LANDSAT images for mapping, inventory and monitoring of reefs. - Serge Andréfouët - Remote Sensing/ Biological Oceanography  University of South Florida, St Petersburg, USA Laboratoire de Géosciences Marines et Télédétection Université Française du Pacifique, Tahiti. 5 km.

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using spot landsat images for mapping inventory and monitoring of reefs
Using SPOT-LANDSAT images for mapping, inventory and monitoring of reefs

- Serge Andréfouët -

Remote Sensing/ Biological Oceanography 

University of South Florida, St Petersburg, USA

Laboratoire de Géosciences Marines et Télédétection

Université Française du Pacifique, Tahiti

slide2

5km

Anaa Atoll (French Polynesia)

SPOT XS-3,2,1

Lagoon

Ocean

Rim

slide3

5km

Marquesas Key (Florida, USA)

LANDSAT 5 TM-3,2,1

slide4

20/30 meters

20/30 meters

slide5

20/30 meters

20/30 meters

slide6

40/60 meters

40/60 meters

slide7

300 meters

300 meters

slide8

Atoll

Lagoon

1500 meters

Ocean

1500 meters

Lagoon

slide9

XS3

XS1

XS2

SPOT IMAGES

Pixel P

XS3

XS3

XS2

XS1

XS2

Spectral space

P

XS1

remotely sensed information
Remotely sensed information
  • LwXSi= LwXSb + LwXSw (+ Lwa)

SPOT: XS1, XS2

LANDSAT: TM1, TM2, TM3

  • LwXSbrelated to the “bottom” features
  • LwXSw related to the water column features
spectral discrimination
Spectral discrimination

Sensitivity TM1

Sensitivity XS1

Sensitivity XS2

minimum discernable unit mdu
Minimum Discernable Unit (MDU)

Size_MDU = PixelSize.(1+2.ErrorLocation)

if ErrorLocation= 1 pixel (pretty good!!)

SPOT MDU= 60 m x 60 m

LANDSAT MDU = 90 m x 90 m

slide17

Minimum Discernable Unit (MDU)

2 x 2 m : not enough

CASI image:

PixelSize= 1 meter

4 x 4 m : ok for training

slide18

Minimum Discernable Unit (MDU)

MDU= 3 x 3 m

MDU= 60 x 60 m

remotely sensed information1
Remotely sensed information
  • Lwi= Lwb + Lww (+ Lwa)
  • 2 or 3 known measurements:

XS1 and XS2

TM1, TM2 and TM3

  • 2 unknown variables Lwb and Lww
slide21

Computed depth

Real depth

Depth

8km

Bathymetric modeling

(Lww)

slide22

Image of thebottom

Scale

“Radiance” scale

architecture forms and dimensions

Branching

Massive

Laminar

Foliaceous

Columnar

Encrusting

Free-living

Architecture (forms and dimensions)

Source: Veron (1986)

hierarchical clustering of the stations

Similarity

Hierarchical clustering of the stations

Reef

Soft Bottom

Hard bottom

Sand/Rubble with

Isolated-Patches

Pure Rubble

Pure Sand

Living

coral

Living

Dead

field stations

slide27

What type of habitat can you map with SPOTwith a good accuracy (70%) ?Depth < 7-8 metersDefinition: coarseMinimum Discernable Unit= 60 meters x metersBoundary analyses

slide28

Spatial structure of a reef system

Transition

Fragmented

Gradient

Abrupt

boundaries

Patches

slide29
A reef is a complex object, but any part of the reef has a membership degrees in each of the classes
  • This membership belongs to [0...1]
  • Mapping of membership degrees: fuzzy classification
is this membership degree useful
Is this membership degree useful?
  • Mapping
  • Habitats boundary analyses
  • Acanthaster planci outbreaks
slide31

Tiahura

Ocean

Land

slide32

Ocean

Motu

Land

Ocean

Membership degree:

Motu

Land

1

0

Ocean

Motu

Land

Fuzzy

classification

Coral

One map for each class of bottom.

Mapping of the degree of membership.

Heterogeneous

Dead structures

slide33

Tiahura

Ocean

Land

slide34

Ocean

Motu

Land

Ocean

Membership degree:

Motu

Land

1

0

Ocean

Motu

Land

Fuzzy

classification

Coral

One map for each class of bottom.

Mapping of the degree of membership.

Heterogeneous

Dead structures

slide35

Tiahura

Ocean

Land

slide36

Ocean

Motu

Land

Ocean

Membership degree:

Motu

Land

1

0

Ocean

Motu

Land

Fuzzy

classification

Coral

One map for each class of bottom.

Mapping of the degree of membership.

Heterogeneous

Dead structures

is this membership degree useful1
Is this membership degree useful?
  • Mapping
  • Habitats boundary analyses
  • Acanthaster planci outbreaks
slide38

Tiahura

2.5 km

slide39

Transitions between bottom types

Coral

Isolated Patches

Land

Sand

Land

Land

Land

1

0

Possibility

measurement

is this membership degree useful2
Is this membership degree useful?
  • Mapping
  • Habitats boundary analyses
  • Monitoring and sampling designs (Acanthaster planci outbreaks)
slide41

Ocean

Land

Location of A. planci infestations in the 80’s (Faure, 1989)

slide43

What about change detection ?

Histograms of bottom-types in XS1 after bathymetric corrections for 2 atolls

slide44

What about change detection ?

  • Problems in calibration and correction of the images:
  • not enough accurate
  • Benthos:
    • Shifts in living communities : ??????
  • Change in sediment cover (hurricanes) : ok
work in the field
Work in the field

Moorea:

20 transects (60m x~1km) for training and control,

6 days,

2 investigators (Yannick Chancerelle, CRIOBE, Moorea),

Semi-quantitative (5%, 15%, 25%, >50%) rapid assessment for 4 variables

Atolls: 20 transects, 2 days, 2 investigators

Caveat: Only assessment of the coarse level of habitat without

hierarchical sampling (if not, time x 10) !!!

work in the image processing lab
Work in the image processing lab

Bathymetric correction

Fuzzy classification to output membership degrees

Mapping of the membership degrees

3days - 1week

Conditions:

- user-friendly software does exist

- good control of the software

- good quality of the data (image and field data)

- skilled analyst (if not, time x 10)

water parameters
Water parameters

Few direct observations.

Potentially interesting for atoll lagoons

(phytoplanctonic biomass or suspended matter)

Many indirect observations

(the water body is not the target)

rivers run-off, pollution, boundary characterization and residence time

slide48

Spatial structure of a reef system

and fluxes

Reka-Reka Tepoto Sud Tekokota

Boundary conditions controls:

Nutrients limitations

Residence time of lagoon waters

Recruitment

Community structure

slide49

Atoll rims typology

Structure

Wave Exposure Hydrodynamic

aperture

aperture

33 %

aperture

> 70 %

South

slide50

Empirical relationships between flows of oceanic water

and wave height for each type of rim

H_Topex (m)

Flows (m2/s)

distribution of coral reefs
Distribution of coral reefs

Global scale

Estimated 255.000 km2 (Spalding & Grenfell, 1997)

slide53

Similarity

Reef

Soft Bottom

Hard bottom

Sand/Rubble with

Isolated-Patches

Pure Rubble

Pure Sand

Living

coral

Living

Dead

field stations

slide54

Global Coverage: NASA plans to collect ~200 LANDSAT 7 images per day worldwide:

    • Long-Term Acquisition Plan (LTAP)
slide55

Present coverage of reefs by LTAP

Expected: one cloud-free image per year

slide56

Present coverage of prioritized reefs by LTAP

(research activities)

Expected: 4-6 cloud-free images per year

slide57

Global coverage

  • Estimation of global distribution of reefs,
  • without ground-truth,
  • 2 classes (soft and hard-bottom),
  • 80% accuracy
  • A basis for extension of monitoring worldwide,
  • 6 classes (gradient of soft and hard-bottom),
  • with ground-truth,
  • 70% accuracy
  • Interface with monitoring organizations is required to get
  • training data for image processing
slide58

Conclusions

Using SPOT-LANDSAT images for mapping, inventory and monitoring of reefs?

Pragmatic point of view

Mapping: Yes:

- Coarse habitats with ground truth

Bathymetric and atmospheric corrections required

- Soft/hard bottoms without ground-truth and corrections

- Boundary analyses

Inventory: Yes

% of soft/hard bottoms: global scale (LTAP):

% of coarse habitats:reef-scale

slide59

Conclusions

Using SPOT-LANDSAT images for mapping, inventory and monitoring of reefs?

Monitoring: Not directly

Change detection generally not possible

Coarse-habitats level not generally a relevant parameter

Water quality generally not directly available

But provide:

Geophysical parameters (exposure, bathymetry, residence time, geomorphology)

Habitat mapping to stratify monitoring and establish new sites

Generalize species indicator at reef scale

Timing: once or variable (catastrophic event)

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