Senegal Land Cover Mapping

Ugo LeonardiFAO GLCN - Land Cover Mapping/Remote Sensing Specialist

ugoleonardi@yahoo.it

•

•Basic dataset

•Ancillary data collected and used

•Photo-interpretation

•Effects of vegetation seasonality

•Field verification campaign & results

•Vedas procedure on Senegal LC

•Land cover spatial aggregation

Presentation Topics

The images used for the interpretation work are:

•A set of 11 Landsat ETM scenes of 2005

•A set of 13 Landsat ETM scenes of 1999-2001

The Dataset

20002005

The Dataset

Ancillary Data

Centre de Suivi Ecologique made available to the project aerial photographs of

Tamba, Kolda, Salemata and Ouli regions, and the interpreted shapefiles of Tamba and

Kolda regions. The legend of the former interpretation was translated in the

respective LCCS classes and the polygons smaller than 5 ha have been eliminated

Before using the Tamba and Kolda shapefiles as reference base for the interpretation,

they have been georeferenced again since they displayed a shift with the 2005 images.

CSE contributed to the land cover mapping, also giving interpreted shapefiles covering

the Dakar area; the one of 1999 was used as base for the interpretation of the Dakar

area. The codes have been translated in LCCS classes and small polygons (< 5ha) were

eliminated.

USGS contributed to the Senegal land cover mapping with 758

geocoded aerial photographs, covering Senegal and Gambia, taken

during the 1994 country aerial survey campaign.

The aerial photos became a reliable reference point during the photo-

interpretation work, since they have been linked with the points of their

position.

Aerial photographs give a better perspective compared to the field

photos, showing effectively the spatial distribution and land cover

features.

Ancillary Data

The original classes of the former 1984 Senegal Land Cover map (scale 1:1.000.000)

have been grouped in major land cover classes.

The vector shapefile was converted in a raster file and georeferenced, giving one more

interpretation tool for the photo-interpretation, since it shows the distribution of the

main types of vegetation covers according to Senegal climatic zones.

Ancillary Data

Ancillary Data

In addiction to the ancillary data collected, the Google Earth

freeware (http://earth.google.com/) gave an extraordinary chance

to photo-interpreters to detect the land cover feature.

Ancillary Data

Photo-interpretation

The implementation of Senegal Land Cover map is

based on the multi-phase image interpretation

approach, which was successfully used by FAO in a

number of projects.

The visual interpretation was carried out using the

GeoVIS software (http://www.geovis.net/), a vector-

based editing system specifically designed for

thematic interpretation.

It is a user-friendly system that embeds the main

tools of vector drawing and editing, including

topological functions, with advanced capabilities

of raster management (Radex) and a direct link

with LCCS (Land Cover Classification System)

software.

The photo-interpretation mapping scale was

1:100,000

During the mapping activities, the GeoVIS “Multiple Windows” tool was used to visualize, at the

same time, the Radex mosaic of both dates 2000 and 2005.

The digitization base was the 2005 mosaic even if it shows, in some portions, black strips due to

the Scan Line Corrector failure, affecting Landsat satellite sensor from 2003 onward. Whenever the

noise caused by the black strips made difficult the interpretation of the 2005 image, then the 2000

one was used as reference base.

Photo-interpretation

200

0

200

5

As concern the visual interpretation, more weight was given to the image showing the driest

situation, in order to avoid an overestimation of the vegetation cover, getting a more reliable

interpretation. In fact, the herbaceous layer presents during the wet season most of the time covers

the reflectance of trees and shrubs, making sometime a difficult task to separate the different natural

vegetation classes.

In Senegal, usually the November date is the best one, since the herbaceous layer is almost dry,

while trees and shrubs still have green leaves.

Photo-interpretation

October 2005 November 1999

Concerning the agricultural areas falling inside the so called Peanut

Basin, it was decided to map the agriculture present in both dates. So,

the agricultural classes displayed on the final interpretation of this

area will show the sum of

the agricultural areas of the period 2000-

2005.

In fact, the whole Peanut Basin is a big

agricultural area, where fields

may have a fallow period that, in this

case, was considered no longer than 5

years. During the fallow period the cover

consists mostly of grass and light bush

vegetation. Natural vegetation mapped inside this area was detected

both in 2000 and 2005 image.

Photo-interpretation

200

0

200

5

In same cases the use of Google Earth produced controversial

interpretations (amended during the land cover revision), due to the

drastic changes in vegetation cover appearance caused by seasonality. This

change is especially marked in woody vegetation which in Senegal normally

is broadleaved deciduous.

It means that woody vegetation, during the dry season, is leafless so if the

acquisition date of the image analyzed corresponds to the dry season, the

woody vegetation almost disappears.

Therefore, the use of the Google Earth high resolution images imply a good

knowledge of the area seasonality, i.e. when both dry and wet season occur,

in order to give a correct interpretation of the vegetation cover

Photo-interpretation

Marc

h

Novembe

r

FEBRUARY 2005

NOVEMBER 2005

Photo-interpretation

One more example of vegetation seasonality effects

On the other hand, the analysis of images with different acquisition dates, in

same cases, gave the chance to determine the extension of flooded areas

and to estimate the water persistence.

Photo-interpretation

Septemb

er

Decembe

r

28

October6

November

At the end of May 2007, after the completition of the preliminary interpretation, a field work campaign was carried out. The steps to organize the field work campaign can be summarized as follow:

1. Detection of the unclear situations encountered during the preliminary photo-interpretation.

2. Identification the area to be checked and the route to follow, according to the accessibility of the points.

3. Uploading of the point to be checked on the GPS.

4. Preparation and printing of a series of maps highlighting both points to be checked and routes to follow.

5. Performing of the field work, compiling the Field Verification Form and taking extra field information.

6. Arranging of the data collected during the field work campaign, in order to be easily accessible for both the photo-interpreters and any final user interested.

Field verification campaign

The field verification work was performed by two groups in the same

period. Each group had the task to reach the points uploaded on the GPS

and fill the Field Verification Form for each point.

The two routes programmed, passed from the following places:

Route 1: Dakar – Thies – Kebemer – Louga – St. Louis – Richard Toll –

Dagana – Salde – Linguere – Dara – Louga – Keur Momar Sarr - Dakar.

Route 2: Dakar – M’Bour – Fatick – Kaolack – Koungheul – Tambacounda

– Goudiri – Kidira – Saraya – Kedougou – Niokolo Koba – Tambacounda –

Dakar.

Field verification campaign

ROUTE 1ROUTE 2

The result of the field verification campaign are a total of 171 point checked along two different Route. For each point a Field Verification form was comipled.

Moreover, 706 extra points have been taken all along Route 2.

Field verification campaign

The data collected was arranged in an Arc View shapefile where both fixed

points and extra points are coded, described and hot-linked with

photographs in an interactive database.

Field verification campaign

The Field Verification Form:

Senegal Land Cover Dataset in numbers

▷ The land cover legend of Senegal, consists of 55 classes and was set

up using the F.A.O. LCCS methodology.

▷ Senegalese full resolution land cover dataset is made of 23,922

polygons, covering an area of 19,659 thousands hectares.

▷ During the field work, 171 field verification forms have been

compiled, and 706 field extra observations (GPS coordinates, a

photo and a short description/code for each point) incremented the

data collected.

Senegal Final Legend consists of 55

classes.

FAO, through Africover Project and Global

Land Cover Network, has developed a

comprehensive, standardized a priori land

cover classifications system (LCCS).

This methodology was applied to shape

the land cover classes of Senegal and

which will be explained in detail later, with

examples taken from Landsat ETM, Google

Earth High Resolution imagery, aerial

photograph and field photos.

Examples of Senegal interpretation detail reached (scale 1:100 000)

Application: testing Vedas procedure on Senegal LC

Vedas (Vegetation Dynamic Assessment) software was applied in East Africa during

2007 GLCN activities and was demostrated that this procedure is able to extrapolate

eco-climatic information on GLCN layer.

In October 2008, the Vedas procedure was tested on the Senegal land cover dataset,

with Modis 005 remote sensed vegetation data (250 mt resolution – 16 days period)

and with Spot vegetation data (1km resolution – 10 day period).

Summarizing, the average NDVI values (calculated in the 2001-2007 period for Modis,

and 1999-2006 period for Spot) was extracted for each polygon of the Senegal land

cover dataset, providing consistent spatial and temporal comparisons of the vegetation

conditions, and monitoring vegetation activity in support of phenologic, change

detection and biophysical interpretations.

NDVI profiles of different land cover classes can differ in mean values but tend to have

a similar shape linked to the seasonality of local vegetation.

Full Resolution vs. Aggregation

F.A.O. data distribution policy, provide for the creation of an aggregated dataset

starting from the original one. For this reason two versions of Senegal land cover

dataset exist:

1. The original full resolution dataset, consisting of 23,922 polygons

2. The spatially aggregated dataset, consisting of 21,238 polygons

The original full resolution dataset was aggregated on the basis of a spatial criteria

rather than a thematic one, producing the reduction of about the 11% of the total

amount of polygons.

Spatial aggregation criteria

The classes listed in the below tables have been aggregated only when occurring as

single units and considering the following spatial criteria: after sorting the polygons

according to their size, for each class was calculated the amount of polygons

corresponding to the 20% of the total

number. The resulting number of polygons

was eliminated starting from the

smallest size forward.

Given that mixed units represent already a spatial generalization, they were

not considered in the aggregation process, except for the classes created

specifically to be used in mixed units, which are:

Spatial aggregation criteria

The above classes are always found associated with other classes in mixed

units, but they have been aggregated using the same procedure explained for

the single units.

The aggregation process was performed with ARCGIS software using the “Eliminate”

extension, after selecting all the polygons to be aggregated. The selected polygons

have been merged with the neighbouring unselected one with the largest area, by

dropping the shared border.

Spatial aggregation criteria

Spatial aggregation criteria

The below single units have not been aggregated:

Spatial aggregation results

Thank You