Data Preparation for Geologic Mapping

Preparing data for a digital geologic mapping project generally involves three steps:

1. Preparing digital base map data (i.e. downloadable or previously stored thematic, topographic, or remotely sensed data, or data that you digitize, scan and georeference);
2. Creating a database and/or individual files to store data that will be gathered in the field (e.g. the locations and descriptive attributes of rock units, rock unit contacts, and measured attitudes);
3. Creating a map that is ready for editing in the field.

What follows is discussion and instructions for completing these steps with ArcGIS 9.x and ArcPad 7.x software.

Design of GeoPad projects can take several tracts, from those that fully implement all of the features of digitizing in a field environment, to those that implement only a subset (e.g. collection of linear (e.g. faults, unit contacts) and/or point (strike/dip, station observations) features; or simply constructing sketch maps on spatially referenced base maps.

The techniques discussed below might be tested first with a pilot project done in a local setting.

The following steps illustrate the preparation of a GeoPad field mapping project, once a field area has been selected (a second option yet to be created, is the download of a map document template and geodatabase for a prototypical map area that can be used a model to be modified as needed):

Image courtesy of Mark Helper.

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Base Map Preparation

Preparation of base map data requires familiarity with GIS data formats and spatial references (i.e. datums and coordinate systems/projections) so that all base map layers properly co-register and have adequate resolution. For base map data obtained from different sources, preparation may, for example, involve projecting datasets to a common spatial reference, defining a spatial reference for data that lack such, clipping to an area of interest and/or preparing derivative base map layers, such as those that portray slope or topography in shaded relief. Detailed procedures for scanning, georeferencing and digitizing paper maps are beyond the scope of this discussion but are available at the GIS and GPS Applications page at the University of Texas.

1. Define region of interest for current project
2. Decide which datasets are valuable to your project
3. Identify sources of datasets and download to local computer
   1. Topographic
   2. Thematic
   3. Imagery
4. Identify and georeference any additional non-digital sources of spatial data
1. Compile tabular data with x,y location information into a spreadsheet and add to GIS application
2. Scan paper map products
1. Clip scanned image to area of interest and save as compatible file format (tiff, jpeg)
2. Georeference scanned image (Example 1, Example 2)
5. Confirm that all datasets are in the same coordinate system, projection and datum. (Example)
1. If coordinate system is undefined, find original coordinate system and establish spatial reference (Example)
2. If not all the same, project to a common coordinate system and datum (Example)
6. Prepare topographic derivatives such as slope or hillshade layers
7. Clip all spatial data to a common project area extent

Databases and Editable Files

Creating a database and/or stand-alone files to store data gathered in the field is less straight-forward and can take several tacts, depending on the degree of sophistication desired, the software available in the office and field, and one's facility with database design and implementation. At the simplest level, the sticky issues of structuring and creating files for field editing could seemingly be avoided entirely by simply drawing geologic line, point, and polygon features directly on base map layers using the tablet drawing tools available, for example, in ArcMap software. This intuitive approach is unfortunately unsatisfactory from several standpoints, not the least of which is an inability to also capture and store descriptive data (a.k.a. "attributes", e.g. strike and dip measurements, inferred vs. exposed contacts, formation names, etc.) associated with each feature that, at a minimum, are needed to symbolize and annotate a geologic map. GIS software is designed with precisely this consideration in mind, and ESRI "feature classes" provide the file structure for storing such attributes for each and every feature mapped. Feature classes that hold digital data come in two flavors: those that reside within a geodatabase environment and those that stand-alone (i.e. shapefiles). Feature classes within geodatabases have two principle advantages over stand-alone shapefiles for field mapping:

1. The capability of applying a "domain" to each attribute to be collected. A domain is predefined list of all possible values for an attribute (e.g. the domain for the attribute "line type" might have the values solid, dashed, dotted, or queried). When entering or editing attribute data, domain values appear as selectable options in a list. Domains can be text or numeric values, like strike, dip, line type, formation name, etc., allowing simple and rapid data entry in the field. A similar ability to pick attributes values from a list can be created for shapefiles, but requires custom forms built with ArcPad Studio or by hand coding.
2. If shapefiles are required, for example for use with ArcPad software, then they can be created from geodatabase feature classes with the domains intact. An export tool is available in ArcMap to create shapefiles from geodatabase feature classes that simultaneously creates forms for attribute entry. The forms contain domain values in selectable lists, again obviating the need to peck in letters or numbers with a stylus. Once edited in the field, shapefiles created this way can be imported back into the geodatabase, updating the original feature classes from which they came.

Project Design

1. Feature classes that hold digital data come in two flavors: those that reside within a geodatabase environment and those that stand-alone (e.g. shapefiles). Feature classes within geodatabases have two advantages: predefined attribute domains allow rapid entry of data either within forms in ArcPad or when editing directly within ArcMap. Below are examples of each of these approaches.
   • Geodatabases
     • Tutorials exist to help you create your own geodatabase
   • Shapefiles
     • Tutorials exist to help you create your own shapefiles
2. Create a map project within ArcMap that contains base map layers and newly created feature classes

• Set the 'Map Properties' for the ArcMap project to always use relative paths to locate data sources.
• Load all base map layers and geologic feature classes into ArcMap
• Create/download symbology (i.e. layer files) for the feature classes line data, point data
• For ArcPad projects, export feature classes and base layers to shapefiles and MrSid rasters with the ArcPad export toolbar in ArcMap. With an ArcMap project that uses geodatabase feature classes with defined domains, this automatically creates data entry forms like those that can be created manually with ArcPad Studio. Clipping data sets to project area boundaries can be done at this stage as well.