ALTIMETRIC & BATHIMETRIC DATABASES

DIGITAL TERRAIN MODEL TECHNICAL NOTE - DTM -

Matrix format

Gridded data model representations of elevation data. The matrix file represent regular X Y nodes. Each node contains a Z elevation value. XYZ data represents either longitude/latitude, or geocentric (x, y, z), or projected (eastings, northings) informations. These data can belong to any coordinate system, in any unit system. Data can take care of any ellipsoid model.

The pace between each node can take any value. It can be less than one meter, or more than 1000 meters. The elevations stored in each node have values that depend on the final application of the client. It will reference either the maximum value of the cell, or a statistical value, or the geodetic value (Z at geodetic location).

The interpolation or extrapolation model used to generate the DTM, depends on the final application. Among the models the user will have access to linear, paraboloid-hyperbolic, fractal, with or without extrapolation.

Terrain can be land, submarine or bottom of streams.

Vector format

Contour lines files in vector format. These lines are generated by the same process. Known elevation points can be put in the file. The precision and roughness of the vectors are tunable depending on the customer's needs.

Technology

We use recognation technology. Our main tool, GeoImage detects automatically the colors of the contour lines on the map. It monitors the operator to give elevations to the lines and points. The interruptions of elevations lines are remade by means of a software. A specifical function allows to draw valley bottoms or ridges in line form to higher slope and ridge lines.

Some XYZ informations which come from different sources - geometer's plottings for instance - can be used.

The elevations into a DTM matrix. Then the interpolation computes the missing nodes. The result is a facsimile of the original map. The result is much more precise than any other technique.

One other advantage is that it is a fast method. The consequences are that we can serve a client in a very short time, and that the cost is much lower.

Accuracy

As mentionned, due to our technique, the result is very similar to the original map. Regarding the map as standard, the precision is :

• in XY : better than +/- 2 paces, that is less than 20 meters on 1/100,00 scale map,
• in Z : for elevations to the lines and points :
  - no difference with the cartographic map information.
  - between two contour lines : better than the half of the distance in height, that can be +/- 1,25 paces on   1/25,000 scale map.

The XY precision applies for adjacent maps as well. The connection is totally invisible.

This precision is compatible with the military precision in formats DTED1 and DTED2.

The precision is very flexible. we can adapt precision to the client precision. The cost for the client depends primarily on the surface of the map we have to process.

For instance a DTM of 100 meters per node can be produced with maps of different scales, 1/50,000, 1/100,000, 1/250,000, 1/500,000. The price is divided by almost four at each scale change. Maybe the client needs a huge zone with a small accuracy and a small zone with big accuracy. We can produce such a patchwork at the better price.

Quality control

Quality control processes are systematically include in the buiding process :
- control of the altitude data captures.
- control of the altitude projections.
- control by succession of DTM profils in pseudo 3D view.
- control by superposing on the computer screen of the raster and DTM, this allows to control :

• projection
• elevation points
• elevation lines,

- control by sounding
- control by delta Zs differences.
- control by altitude parts and empty zone setting off.

Geographic coordinates

The DTM are delivered in any of the projections or references of the map. It can be UTM, DMS, Gauss or any exotic system or Datum as long as the values are visible on (or inferable from) the map.

Formats

The DTM is supplied in standard formats, i.e DTED, IGN France, Intergraph Raster File, DXF, and others.

Except for DXF, they mainly consist of matrix on X lines and Y columns.

Medium

We can support standards media such as diskettes, DAT, DC6xxx, DC2xxx and CD-ROM. Data can be written for DOS.

COMMERCIAL NOTE

- List of products already existing in the catalog. Technical notes and prices by contact.
- For the products not available in the catalog, we provide quotation.

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