The Direction of Lava flows of a Volcano on Venus

Michael Boyce

Ce394K

Fall 2002

Contents:

Introduction_______________________________________________________3

Methods__________________________________________________________4

Results__________________________________________________________10

References_______________________________________________________11

Introduction:

The surface of Venus is a chaotic and violent place. Eruptive events occur on massive scales, with enough material flowing out to completely resurface the face of the planet. The direction in which the lava flows down the side of the volcano can give indications as to the order of the eruptions and clues as to where the next eruption will take place and to the scale of eruption in any particular volcano. By mapping the lava flow channels it is possible to interpret the order of the eruptive event at any particular site.

Methods:

The project was divided into 6 major steps.

1: Finding the data

2: Analysis of the metadata

3: Selection of a site for digitizing

4: Creating a layer to digitize into

5: Digitizing the layer

6: Analysis of the results

1: Finding the data:

The process of finding the data involved numerous searches on the web for Venus coverages. After many false leads an email to “webgis” produced results. The information was free to use and was in the process of being reprocessed to produce a higher resolution data set.

The data was collected from http://webgis.wr.usgs.gov/venus.htm.

2: Analysis of the metadata.

This step was used to make sure that the data being imported into ArcMap was of a usable type and that it did not contain any surprises. From the metadata the map was shown to already be in an equal distance projection which is what is needed for accurate distance measurements on the “drainage” coverage. Also the metadata showed that the data was in a strange projection where the east and west values were reversed, that is the east measurements were negative and the west measurements were positive. This is counter to the standard earth datum.

3: Selection of a site for digitizing

This is one of the most important steps in the initial processes of the project. The site to be digitized can not be too close to the edges of the projected map, because at these locations the accuracy of the data can not be assured, since the projection of sphere onto a flat sheet means that there will be distortions at the edges of the map. The site also was not to be to complex for the first effort at digitizing. So a site was selected that was of reasonable size and in a location that favored the map projection in it’s location away from the edges.

4: Creating a layer to digitize into

To do this step, the nature of the data to be digitized into must be realized. Since the data is going to have linear features then a line or polyline coverage is desirable. Also since the coverage is going to be used to convey a flow direction it will need to be able to handle the data, the M direction.

Using ArcCatalogue to create the layer, open ArcCatalogue and select the directory that the layer will be created in.

Select File->New->Shapefile

This will open a window that will ask for some features to be defined for the new shapefile.

In the “Create New Shapefile” window, name the layer, select the “Feature Type” and make sure that the tick boxes for the M values are ticked. Then the Spatial Reference must be defined. To do this click on the edit button and import the spatial reference frame from another dataset that is going to be used. In this case it was the elevation dataset. To finish the creation of the layer click “OK”.

This generates a new shapefile layer in the folder with the name and spatial reference selected.

To digitize the “drainage” data the elevation dataset was loaded into ArcMap. The color of the image was changed to produce a more visible difference in the heights of the data.

Since the image washed out in the higher elevations the incorporation of the slope data was used to bring out the smaller features. The slope data was used in grayscale to add contrast and was placed on top of the elevation data. The slope data was then made 50% transparent. This gave the effect of adding contrast in the washed out areas of the map.

The location selected for this report (highlighted in red) was zoomed to increase the accuracy of the digitizing.

To digitize onto the “drainage” layer first add the layer to the ArcMap project then using the editor tool bar select start editing. This activates the editor tool bar and enables the “add line” tool. Using this tool digitize the drainage. Make sure to save the edits frequently.

Results:

The results of the digitizing were quite good and produced a location for the major aperture of the volcano (dark red circle), and a previous eruptive center (blue square). The sequence of eruptions can be determined by the flow paths, one of which flows through the center of the other. The flow direction is what is expected, down the side of the volcano and onto the plain.

Bibliography:

http://webgis.wr.usgs.gov/venus.htm