Post on 14-Jan-2016
description
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