Topological   spatial   data,   linked   to   the   thematic   data   relating   to   vegetation   types   (flora,   ecology,   production),   are   managed   in specific   layers,   all   of   which   are   derived   from   a   primary   layer   called   VEG4.   This   layer,   which   describes   the   boundaries   and   the   spatial distribution   of   vegetation   types,   is   made   up   of   14   535   polygons,   which   are   areas   corresponding   to   the   various   vegetation   types   (with the   exception   of   5   polygons   corresponding   to   open   water   -   the   minor   beds   in   Niger,   Bani,   part   of   the   Diaka   and   the   lakes   -   and   731 polygons   corresponding   to   built-up   areas).   Each   unit   is   made   up   of   a   polygon,   bearing   an   acronym   designating   its   vegetation   type which may be a plant combination or a mosaic of plant combinations. VEG4 ,   the   essential   information   layer   in   the   GIS,   derives   directly   from   the   CIPEA/ODEM   rangeland   map   realized   by   Pierre Hiernaux   and   Mark   Haywood,   who   also   played   a   key   role   in   the   realization   of   this   layer   of   GIS   information   by   performing   the digitization   of   the   map   (30   sheets   40x25   cm   to   1:   100   000)   on   an   A3   format   digitizing   table,   using   the   drawing   software   programme (Autocad). These   maps   were   captured   within   a   5x5   km   squares   grid,   rigorously   using   the   method   developed   by   Mark   Haywood   for   the realization   of   the   original   maps   constructed   on   a   UTM   grid   (spindle   30)   at   a   5   km   step,   each   digital   file   containing   six   5x5km squares, usually arranged in a 3x2 format. Mark   Haywood   was   also   careful   to   note   in   a   text   file,   for   each   polygon   with   a   recognition   mark,   the   unit   acronym   and   the   mark coordinates   in   x   and   y.   This   file,   which   is   independent   from   the   163   DXF   graphics   files   that   represent   the   inland   Delta   rangeland map   (approximately   978   squares   of   5x5   km),   contains   31,083   lines   (600   pages   of   word   processing).   This   data   base   entry   in   a standard   exchange   format   (DXF   format)   cost   Mark   Haywood   three   years   of   work   and   greatly   facilitated   our   task.   However,   the   data from   these   163   files   had   to   be   translated   into   a   single   ARC/INFO   layer   and   a   number   of   methodological   and   technical   problems   had to be solved. For this we transcoded the 163 files into 163 ARC/INFO layers, then merged them into a single layer. There   were,   however,   several   problems   to   be   solved:   a   unit   corresponding   to   a   vegetation   type   is   an   object,   therefore   it   must   be represented   by   a   polygon.   However,   the   presence   of   the   construction   grid   could   lead   to   the   division   of   one   area   into   two,   three,   four, or   more   adjacent   polygons   that   had   then   to   be   merged   back   into   a   single   polygon.   On   the   other   hand,   Autocad   does   not   allow   the transfer   of   unit   attributes,   in   particular,   the   acronyms   identifying   the   vegetation   types.   To   do   this,   a   VEGPOINT   layer   was   created from   the   text   file   including   the   x,   y   coordinates   and   the   acronym   of   the   labeled   point   inside   each   polygon.   We   therefore   had   a   layer containing   all   the   arcs   i.e.   the   faces   of   the   future   polygons,   and   all   the   points   destined   to   become   the   labels   of   the   polygons representing   the   vegetation   units.   Yet   it   was   imperative   to   have   one   single   label   per   polygon,   with   the   right   acronym   identifying   the vegetation   type   of   the   polygon   (in   total,   31,083   points   and   as   many   elementary   polygons).   The   position   and   the   acronym   of   31,083 points   then   had   to   be   checked,   a   process   which   took   several   months,   and   the   adjacent   polygons   bearing   the   same   acronym   were aggregated to get the final layer (Veg1). From VEG1 to VEG4: simplifying thematic information VEG1   contains   14,605   ​​polygons,   each   identified   by   an   identification   number   (ID)   and   an   acronym   referring   to   its   vegetation type,    i.e.    a    total    of    209    different    acronyms,    and    a    number    indicating    the    occurrences    of    the    acronym.    A    number    of    these abbreviations   are   "script"   errors   made   when   drawing   maps   in   Addis   Ababa.   For   example,   the   acronyms   “TA/Z”   or   “TD/B”   indicate mosaics   that   do   not   exist.   In   some   cases,   a   particular   unit   is   given   two   acronyms   representing   the   same   vegetation   type,   one belonging   to   the   inland   Delta   nomenclature   and   the   other   to   the   nomenclature   used   in   the   «dead   Delta»,   for   which   another   vegetation map   was   drawn   (and   has   remained   unpublished)   ...   On   the   other   hand,   some   of   the   mosaics   are   ecologically   "unlikely"   and   are   only represented in very few occasions : 35 mosaics are represented only once, 19 only twice, and so on. In   general,   we   considered   that   all   mosaics   represented   in   10   units   or   less   should   be   replaced   by   the   ecologically   closest   mosaic. This   transformation   also   extended   to   mosaic   considered   unlikely   although   present   in   more   than   10   units   (VSP/P,   AG/TT,   BP/B). Finally,   the   TB/TC   mosaic   (made   of   two   dry   woody   savannas   on   “ togge ”,   very   close   to   one   another),   indicated   in   95   units,   was simplified   as   TB.   The   TB/TC   mosaic   was   attributed   to   “TOPO”   units,   which   represent   the   built-up   area.   This   convention   makes   it possible   to   suppress   “TOPO”   from   the   vegetation   map   (by   replacing   it   with   a   strongly   anthropized   woody   savanna   TB/TC   without losing the polygons which can at any time regain their status as built-up surfaces. Table 1 :

Ultimately,   we   retained   121   vegetation   types   (including   RIVER   which   represents   open   water   and   TB/TC   that   represents   built-up areas).   The   acronym   modification   leading   to   the   121   vegetation   types   is   a   large   simplification   (from   209   to   121   vegetation   types). However,   the   resulting   loss   in   spatial   information   is   very   limited,   with   a   reduction   in   number   of   polygons   from   14,605   to   14,535. This final   layer   is   called   VEG4   and   becomes   the   main   layer   of   the   GIS.   It   has   two   functions:   1)   to   document   the   spatial   distribution   of   the vegetation   types;   2)   to   produce   derived   layers,   FLORE1   for   flora,   CRUE   3   for   ecological   data,   SOIL   for   soils   data   and   PRODUCT   for rangeland fodder production.

F i gure n° 1 :  The process leading to the organization of ARC/INFO covers to create VEG4

GIS organization (1)   SPATIAL DATA CREATION