Geology of Madagascar (1:500,000 Scale)
Go Spatial's
Madagascar Geology map was digitized from the Madagascar Geological Services' Geological map compiled in the late 1960's and 1970's by Henri Bessairie. The original map was published in
French and drawn in the Laborde projection and totals 8 map sheets. Go Spatial Limited translated the map into English and re-projected the map to geographic (lattitude/longitude) coordinates.
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ArcView/ArcGIS Map Interface (Click to enlarge)
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Map zoomed to 1:500,000 scale (Click to enlarge)
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The 1:500,000 Geology package consists of geology polygons, zones of mineralization and crystallization, and linear features such as faults, trends, mineralized veins, etc. The table below details the layers comprising this package the number of features:
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Layer
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Features
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Type |
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Geology Polygons
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4273
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Area |
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Zones of Mineralization
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141
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Area |
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Mineralized Veins
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4066
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Line |
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Faults
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821
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Line |
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Plongements
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814
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Line |
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Gross Anomalies
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5
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Line |
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Escarpments
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20
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Line |
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Line of Crystallization
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7
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Line |
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Total
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10155
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Geo-referencing was carried out by scanning the original paper maps at 600dpi resolution and then placing a control point at each map-grid intersection. Nearly 3,000 control points were used in all. The scanned maps were then digitized and edge-matched.
The 1:500,000 scale geology has a more complicated database structure than our 1:1,000,000 scale product because of the complexity of the map legends on the 8 map sheets. Consult the readme file for details on querying the various attributes in ArcView 3.x and ArcMap 8.x.
Three tables comprise the 1:500K products' database. The main attribute table has the following fields:
- Geotype - Code assigned to each legend element.
- Group - Name of group to which each legend element belongs, if it exists (e.g. Beforona Group).
- Series - Name of series to which each legend element belongs, if it exists (e.g. Beforona, Alaotra, Adriamena, Series).
- Complex - Name of complex to which each legend element belongs, if it exists (e.g. Grano-Dioritic Complex).
- Notes - Ancillary information
- Fossils - Indicates whether surface type is fossiliferous
The second table contains information detailing the surface materials for each legend entry. It has a one-to-many relationship with the main attribute table and has the following fields:
- RTID - Sequential number assigned to each table record.
- Geotype - Code assigned to each legend element. It is used as the relate-item to the main attribute table.
- Surftype - Surface type of each legend element. In the 1:500K map legend, a single legend element can represent many surface types (e.g. sandstone, limestone, marly)
- Location - A village, township or other area name associated with each surface type, if it exists.
- Direction - Indicates whether a given surface type is in the northern, western, eastern or southern part of the geology polygon. Very few features actually have this attribute assigned, but the information was captured anyway where it was present in the map.
- Eon - Geological eon
- Era - Geological era
- Sub-era - Geological sub-era
- Period - Geological period
- Stage - Geological stage
- Class - Sedimentary, igneous, metamorphic (Migmatites were classified as metamorphic except where the presence of igneous rocks were indicated. In these cases migmatites were classified as igneous.)
The third table lists fossils found in each surface type, if any, and has the following fields:
- FID - Sequential number assigned to each table record.
- RTID - Code assigned to each surface type. It is used as the relate-item to the surface type details table.
- Fossil - Type of fossil found in this surface type, if any. One or more fossil types may be present in each surface type associated with each legend element.
Click here to view complete list of fossils present in this table.
The three tables represent a "semi-normalized" database design. The database design was kept as simple as was practical to minimize the impact on users not using ESRI software, specifically ArcView or ArcGis.
Users of non-ESRI mapping software will need to manually establish the table relationships described above.
The 1:500K product features a number of improvements over the 1:1M map:
- Improved French to English translation
- Improved positional accuracy due to the larger number of map grid-intersections
- Much More detail
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