Geologic Line Standards

OK, this is a long one, but I wanted to illustrate a geodatabase coding scheme for various line types used in geologic mapping with ArcGIS. This rather large subset and the related explanations is drawn from my mapping project on the Owyhee River so it is somewhat specific.

The codes are based on terminology in the new digital geologic map standards published by the FGDC. The underlying scheme is based on one developed by Hastings and Sylvester. The seemingly overly detailed list is based on degrees of certainty relative to two aspects of lines on a geologic map: 1. What sort of line it is and how certain you are about that; and 2. How well the line's location is known.

Each funny looking code is a combination of the following characters that account for a variety of lines and a variety of degrees of certainty about what and where they are:

Line Types [kind]

• C Contact
• X Fault
• R Rock body (marker bed or key bed)
• Z Scarp (as feature, not contact)
• M Morphologic
• B Boundary

Prefixes [category]

• g generic
• l landslide
• i internal
• f fluvial
• v volcanic
• s sedimentary
• z scarp
  
• d depression
• m morphologic feature

Suffixes [location]

• c certain
• q questionable
• a accurate
• x approximate
• c concealed
• i inferred

Code followed by Name

• uB Boundary—undifferentiated
• mB Boundary—mapsheet
• pB Boundary—property
• sB Boundary—scratch
• wB Boundary—water
• eB Boundary—exclusion
• gCca Contact—Identity and existence certain, location accurate
• gCqa Contact—Identity or existence questionable, location accurate
• gCcx Contact—Identity and existence certain, location approximate
• gCqx Contact—Identity or existence questionable, location approximate
• gCci Contact—Identity and existence certain, location inferred
• gCqi Contact—Identity or existence questionable, location inferred
• iCca Internal contact—Identity and existence certain, location accurate
• iCqa Internal contact—Identity or existence questionable, location accurate
• iCcx Internal contact—Identity and existence certain, location approximate
• iCqx Internal contact—Identity or existence questionable, location approximate
• sCca Incised-scarp sedimentary contact—Identity and existence certain, location accurate.
• sCqa Incised-scarp sedimentary contact—Identity or existence questionable, location accurate.
• sCcx Incised-scarp sedimentary contact—Identity and existence certain, location approximate.
• sCqx Incised-scarp sedimentary contact—Identity or existence questionable, location approx.
• ldCca Sag-pond or closed depression on landslide (mapped to scale)
• viCca Contact separating individual lava flows within same map unit—Identity and existence certain, location accurate
• viCcx Contact separating individual lava flows within same map unit—Identity and existence certain, location approximate
• viCqx Contact separating individual lava flows within same map unit—Identity or existence questionable, location approximate
• gXca Fault (generic; vertical, subvertical, or high-angle; or unknown or unspecified orientation or sense of slip)—Identity and existence certain, location accurate
• gXqa Fault (generic; vertical, subvertical, or high-angle; or unknown or unspecified orientation or sense of slip)—Identity or existence questionable, location accurate
• gXqx Fault (generic; vertical, subvertical, or high-angle; or unknown or unspecified orientation or sense of slip)—Identity or existence questionable, location approximate
• gXcc Fault (generic; vertical, subvertical, or high-angle; or unknown or unspecified orientation or sense of slip)—Identity and existence certain, location concealed
• kRca Key bed—Identity and existence certain, location accurate
• kRcx Key bed—Identity and existence certain, location approximate
• fZca Fluvial terrace scarp—Identity and existence certain, location accurate. Hachures point down scarp
• fZqa Fluvial terrace scarp—Identity or existence questionable, location accurate. Hachures point down scarp
• fZcx Fluvial terrace scarp—Identity and existence certain, location approximate. Hachures point downscarp
• lZca Head or main scarp of landslide—Active, sharp, distinct, and accurately located. Hachures point down scarp
• lZcx Head or main scarp of landslide—Inactive, subdued, indistinct, and (or) approximately located. Hachures point down scarp
• liZca Internal or minor scarp in landslide—Active, sharp,distinct, and accurately located. Hachures point down scarp
• liZcx Internal or minor scarp in landslide—Inactive, subdued, indistinct, and (or) approximately located. Hachures point down scarp
• vMca Flow lobe or lava-flow front—Identity and existence certain, location accurate. Hachures on side of overlying younger flow
• vMqa Flow lobe or lava-flow front—Identity or existence questionable, location accurate. Hachures on side of overlying younger flow
• vMcx Flow lobe or lava-flow front—Identity and existence certain, location approximate. Hachures on side of overlying younger flow
• vMqa Flow lobe or lava-flow front—Identity or existence questionable, location approximate. Hachures onside of overlying younger flow
• vMm Crest line of pressure ridge or tumulus on lava flow

Mandatory (Basic) Image Enhancements

Smooth Your Image:


To get the most most out of your base imagery, you need to experiment with different image enhancement tools in Arc. The basic manipulations can be found under the 'display' and 'symbology' tabs found under 'layer--properties' (right-click on the layer of interest). To smooth the image without any negative effects, choose the 'bilinear interpolation' option and then click 'apply'. This will smooth your image in a visually satisfying way. Other resampling options may result in bothersome artifacts in the typical types of imagery that geologists use for mapping.

Stretch Your Imagery:

Stretching your image can create levels of contrast and color balance that you will appreciate. For details, consult a remote sensing textbook. For now, just accept the fact that you can vastly improve an image's appearance by applying a standard deviation stretch to your data. Start with n=2 and experiment with increasing and decreasing this value. Also, if you limit the stretch statistics to the 'Current Display Exent' you will get ~local results that typically improve the contrast of the image. This will vary with the absolute range of values present in the current display. Experiment with other stretches.

Both of these enhancements are useful for b/w DOQQs, color DOQQS, and Quickbird data among (presumably) all other remotely sensed base (photo-like) imagery. It is not useful for DRGs.

Geologic Mapping Toolbar in ArcGIS

If you are limited to working with one screen, you will find it useful to make a custom toolbar to assist in compiling a geologic map. Simply right-click on the toolbar area and scroll through the list of available toolbars to the word 'customize', then make and name a new toolbar. You can then drag individual tools (commands) from the long lists of possibilities onto your new toolbar. In some cases, you will find that you have to drag the tool from an existing bar on your screen. The scale tool is an example that comes immediately to mind. The shot above is from my laptop.

Geologic Time and Symbol Standards

It may interest you that there is a newly minted geologic time scale and implied color scheme recently released by the USGS and the AASG:

http://pubs.usgs.gov/fs/2007/3015/

Also note that there is a comprehensive document outlining a digital cartographic standard for geologic map symbolization:

http://ngmdb.usgs.gov/fgdc_gds/

At NBMG and other agencies, the implied color scheme in the timescale is largely unworkable in detailed maps of Tertiary and Quaternary deposits.

As for the symbology, this is an obvious standard for all geologic mappers and agencies to adopt.