Geology of Madagascar

Introduction

The Geology of Madagascar data products come in two flavors, 1:1,000,000 scale and 1:500,000 scale. Both maps were digitized from paper maps series published by Henri Bessaire between 1957 and 1972. A total of three maps series comprising 22 map sheets were scanned, georeferenced, edge-matched and digitized to create the digital products available from Go Spatial Limited.

All the maps were translated from French to English by our staff. Thousands of control points derived from lat/lon grid intersections and selected features from our Vector Base Map were used to re-project the scanned maps from the Laborde projection to geographic coordinates.

Go Spatial Limited staff visited Madagascar between April and June 2004 to make a cursory assesment of the maps' accuracy. Using "windshield" ground-truthing techniques with a GPS receiver and a laptop computer, we can confirm that boundaries between geology polygons fall in the transition zones between rock units and geological classifications. Observations were made between Antananarivo and Tulear, Antananarivo and Majunga, Ambanja and Diego Suarez and also in the Brieville area in Toamasina province. Obviously it is not possible to ground-truth the entire island, and whatever innacuracies inherent in the source materials are present in the digital data.

Getting Started

After downloading your data from the Go Spatial website, unzip the archive to your C: drive. A directory will be created called GoSpatialData. Under this directory will be a subdirectory named after the package you downloaded.

Example: V-AFR-016.zip

C:\GoSpatialData\AFR-016\

Another subdirectory will be created named after the product code of the package you purchased.

Example:

C:\GoSpatialData\AFR-016\V-AFR-016

In this directory you will find two files, one with an .apr extension and another with an .mxd extension. These are ArcView and ArcMap project files respectively. If you have either of these programs installed on your computer, click on the project file and you will be presented with an initial view of your data. Note that this view is merely meant as a starting point for people not familiar with GIS software. The view is not meant as an example of proper cartographic design.

The data subdirectories are located under the directory named after the code for the product you purchased. They are Areas, Lines and Points. The Areas subdirectory contains ShapeFiles for layers of polygon features. The Lines subdirectory contains ShapeFiles for layers of line features. The Points subdirectory contains ShapeFiles for layers of point features. Sample data packages differ slightly in their file organization, but the above explanation should make clear the packaged files organizational structure.

Example:

C:\GoSpatialData\AFR-016\V-AFR-016\Areas
C:\GoSpatialData\AFR-016\V-AFR-016\Lines
C:\GoSpatialData\AFR-016\V-AFR-016\Points

For information regarding technical details and an explanation of the database columns and their meanings, go to the Go Spatial Limited website and look up the metadata for an individual layer. The Entity/Attribute section of the metadata file provides an explanation of the database columns and their meanings.

Attributes

The attributes of the 1:1,000,000 scale geology are stored in a single .dbf file. Attributes for the features in all layers can be queried in the normal manner using the identify tool in ArcView and ArcMap or its equivalent in other GIS software. The attributes of the geology polygons in the 1:500,000 scale product are stored in three separate tables:

• geology.dbf
• geotype.dbf
• fossils.dbf

The geology.dbf is the main attribute table associated with the ShapeFile for the geology layer. The geotype and fossils tables hold extra information about each surface type. These tables are necessary as the way the legend on the paper maps is organized represent one-to-many ralationships between geology.dbf -> geotype.dbf and geotype.dbf -> fossils.dbf. For details of the columns definitions and the relationships among tables, see http://www.gospatial.com/html/madagascar_500K.php.

Querying Attributes

In ArcView 3.x, attributes are queried using the feature selection tool. Using the the identify tool in ArcView 3.x will only bring up the attributes stored in the geology.dbf file. This table merely holds the unique identifying code for each cover type as well as some other data unique to each cover type. The descriptions of the cover types represented by each polygon are stored in the geotyp.dbf file. The fossils.dbf file shows the fossils present, if any, in each cover type described in geotype.dbf.

Screenshot showing the attributes for a polygon selected using the feature selection tool in ArcView 3.x
The feature selection tool is the depressed button on the toolbar above the map image.

The screenshot above shows a portion of the 1:500,000 scale geology polygons displayed in ArcView 3.x. The three linked attribute tables are displayed to the left of the map image. The rows highlighted in yellow are the records in each table associated with each polygon. In ArcMap 8.x, querying attributes is a bit simpler. The identify tool in ArcMap is more robust than in ArcView 3.x and is able to show all attributes from related tables in a hierarchical tree structure.

Screenshot showing the attributes for a polygon selected using the feature selection tool in ArcMap 8.x
The feature selection tool is the depressed button on the toolbar the left of the displayed attributes.

Note the organizational structure of the attributes for the selected polygon. The code cf69 is the unique identifier for the surface type represented by the slected polygon and is stored in the main attribute table, geology.dbf. Associated with cf69 are two records in the geotype.dbf file. These are the surface types represented by the selected polygon. The highlighted entry, 313, shows the attributes for that surface type.

Screenshot showing the fossils present in the selected polygon.

The screenshot above shown the fossils present in the cover type detailed in the previous example. Simply using the identify tool in ArcMap 8.x enable the user to easily explore all the information associated with each geology polygon. Queries can also be designed around the related tables enabling sophisticated cartographic modeling that would be impossible with the original paper maps.

Satellite Band Combinations (Geology sample data package only)

The ArcView 3.x and ArcMap 8.x project files included with the geology sample data package are display a three-band combination of a Landsat TM5 image. For the 1:500,000 scale geology, the following combination is used:

• Band 4
• Band 6 (Landsat Band 7)
• Band 2

• Band 4
• Band 5
• Band 1

Right-click on the Landsat TM5 legend entry and select Properties to bring up the above screen in ArcView 3.x

More details regarding each data product can be found on the respective product pages. Navigate to our home page and click the Madagascar link to access the individual product descriptions. We hope you find our data useful and we welcome any comments, suggestions or questions you may have. Thank you!