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By Mike Price, Mining Industry Solutions Manager
Editor's Note: This article describes how to properly rotate symbols in ArcView GIS by applying real-world compass measurements. Visit the "Terrain Modeling with ArcView GIS" page on this Web site to read previous articles that describe downloading, converting, and modeling with various types of data Digital Line Graphics optional format (DLG-O) vector data, digital elevation model (DEM) raster data, and Geographic Names Information System (GNIS) label points--available from the U.S. Geological Survey (USGS). This Web page includes sample data sets for previous articles, conversion utilities, and links to data sites on the Web.
The sample data for this article, called placitas.zip, can be downloaded now. After extracting the 13 files for this exercise from the zipped file, retain the zipped file in case you want or need to redo some or all of the steps described here. Once the symbols in a project are rotated, they cannot be unrotated.
More Uses for Symbology
Many industries are using more complex symbols for representing point data sets. To accurately depict this data, some or all of the symbols may need to be rotated. This tutorial uses a data set that includes sedimentary Strike and Dip, metamorphic foliation, jointing, and other types of point data that would be used by geologists and others for structural modeling. This data set is based on the USGS Placitas New Mexico 7.5-minute quadrangle and contains a point file and associated geology polygons.
The sample data can be combined with other data available from the USGS site for the Placitas area such as DLG-O, DXF, and GNIS data. The project data is Universal Transverse Mercator (UTM) projection North American Datum for 1927 (NAD27) Zone 12. Any additional data needs to be in or reprojected into UTM NAD27 Zone 12. The data set includes a point data file called PLACPNT1 and a polygonal geology file called PLACGEO1. Create a directory for this project call PLACITAS. Unzip and place all the files for this project in the PLACITAS directory.
The Geologic Symbol Palette
Both ArcView GIS 3.1 and 3.2 include a palette of selected geological point and line symbols. This marker and pen palette, named geology.avp, as well as many other special-use symbolsets for forestry, transportation, and other disciplines, come with ArcView GIS. The symbols in the geology set represent many types of geologic point and line features as defined by the USGS in the Open File Report 95-525 and its successors that are the current standards for geologic symbology.
The palette includes 108 marker symbols for minor geologic point types and 168 line types for representing major geologic line features. The point symbols were created as TrueType fonts so they can be scaled and rotated. Line Symbols that have directionally sensitive crossing symbology are duplicated in the palette so changes in line direction can be correctly digitized.
Field observation of geologic point sources often includes the following measurements:
- Strike or trend Azimuth or Bearing
- Crossing orientation (e.g., Dip direction
- Crossing magnitude (e.g., Dip angle)
Points are plotted on maps to represent linear and crossing geometry, and they are often attributed with a value showing the size or magnitude of Dip. Point symbology may be rotated in an ArcView GIS view to display the true orientation of field data. Field geologists and surveyors use one of two methods. Azimuth and Bearing. to measure and record angular field relationships.
Azimuth
Measurements begin at north with 0�, rotate clockwise 90� to the east, then on to 180� due south, next to 270� to the west, and return to 360� at north. The 360� system of Azimuth is shown here.
BearingBearing divides the 360� compass Azimuth into four quadrants of 90� each, named northeast (NE), southeast (SE), southwest (SW), and northwest (NW). Angular rotation in each quadrant is measured from the vertical axis (the north-south compass axis). The table and illustration shown here summarize the concept of Bearing and the direction of measurement for each quadrant.
Field Mapping Issues
Several important issues must be considered when collecting and recording geologic data. Both Strike orientation and Dip Direction must be represented. Strike is a bidirectional linear measurement describing the intersection of a plane and a horizontal surface. Dip Direction represents the steepest line of fall on the horizontal measured plane; it is the path a marble, influenced by gravity, would follow as it rolled down the plane surface. Dip Direction is a unidirectional measurement and is always perpendicular to Strike.
The illustration shown here explains the
relationship between Strike and Dip.
To simplify field measurements, Dip Direction is often combined with Strike by applying the Right-hand Rule. To use the Right-hand Rule when measuring Strike with a compass, always keep the Dip Direction to the observer's right, as though he was looking at his right hand, palm side up, and his right thumb was pointing to the "down-Dip" side. Using this method of recording data will consistently record the Dip Direction in a clockwise rotation relative to Strike. The figure above demonstrates the application of the Right-hand Rule.
If angles are intentionally or accidentally measured using the opposing Left-hand Rule, they can be shifted by adding or subtracting 180� to the Left-hand Azimuth or by swapping north for south or east for west with a Left-hand Bearing.
Surveyor's Trick
To obtain a back-sight Azimuth or to reverse your path of travel and head home, add 180� to a forward compass Azimuth that is less than or equal to 180�, or add 180� to an Azimuth that is greater than 180�. If you use Bearings, swap the N for S and E for W in the Bearing description (or vice versa), and you will be headed back home. For example, to obtain a back Azimuth for an Azimuth of 227� (generally southwest), subtract 180� to calculate 47� (generally northeast). To reverse a Bearing of S82E (slightly south of east), swap the S with an N and the E with a W to obtain N82W and return toward the west.
When recording an orientation using Bearing measurements,
a cardinal direction may be recorded in two ways (e.g., north can be shown
as N00W or N00E). Although the method described in this article
will rotate symbols using data with either Bearing measurement notation
style, the abbreviations for cardinal orientations shown here are
recommended.
ArcView GIS and Angles
ArcView GIS uses the Radial Coordinate System to manage angular measurements and to orient data. In this system, 0� is oriented to the right along the positive x-axis. Positive angular rotation proceeds clockwise 90� to the positive y-axis, then to 180� on the negative x-axis, then to 270� to the negative y-axis, returning to 360� on the positive x-axis. The Azimuth and Bearing systems may be related to the Radial Coordinate System through complex radial mathematics to convert field data to values used by ArcView GIS to rotate geologic point symbols.
A conversion table for ArcView GIS in dBASE format named AZCONV.DBF has been included with the sample data to simplify the rotation process. Joining field data, recorded as Azimuth or Bearing, to the AZCONV table will assign proper ArcView GIS rotation angles to the field data points.
The table contains 365 records, one for each whole number of degree measurement from 0� to 360�, plus four additional records to include alternate Bearings for north, south, east, and west.
Using the Conversion Table
Following is a step-by-step example of how to use the conversion table to rotate point data.
- With the unzipped sample data and AZCONV table in the PLACITAS directory, begin a new
ArcView GIS project and add a new view but don't add any themes.
Load the geologic symbol palette by choosing Windows >Show Symbol Window from the menu bar. Click on the Palette icon located in the upper right
corner of the dialog box. In the next dialog box, click the Load button
and path to the geology.avp palette, which typically will be located in
the ARCVIEW\SYMBOLS subdirectory in the location where ArcView GIS was
installed. After loading the palette, click on the marker button icon
and scroll down to verify that the geologic point symbols have loaded.
- Add the PLCAPNT1 and PLACGEO1 shapefiles, stored in the PLACITAS
directory, as themes to the view.
When displayed, these themes should
have custom legends that have already been classified. The PLACPNT1
theme has the correct geologic symbol assigned to each type of data.
Notice that all these point symbols are oriented in an east-west
direction, and the Dip is pointing to the north. This does not
accurately represent the data and needs to be corrected.
- Make the PLACPNT1 theme active and display its table. Verify that the fields for
Strike and Dip data are present and determine if the measurements are in
Azimuth or Bearing. In the PLACPNT1 table all measurements in the Strike
field are in Azimuth.
- In order to use the AZCONV table to correct the
orientation of the symbols for the PLACPNT1 theme, the AZCONV table must
be joined to the table for the PLACPNT1 theme. Go to the project window,
highlight Table, click the Add button, and path to the PLACITAS
directory containing AZCONV.DBF to add it to the project.
- With both the AZCONV and PLACPNT1 tables open, make AZCONV active and select the Azimuth field. Next make the PLACPNT1 table active and select the Strike field. Choose Tables > Join from the menu bar to join AZCONV to PLACPNT1. AZCONV should close, and PLACPNT1 will contain the fields from both tables. Inspect the Join to verify that all point records have a corresponding conversion. When joining data sets that use Bearing measurements, carefully check north, east, south, and west points to verify that they are complete and remember to enter north as N00E, east as S90E, south as S00W, and west as N90W.
- Now the Strike and Dip
symbols can be properly oriented to the mapped points.
Return to the
view and open the Legend Editor for the PLACPNT1 theme by double
clicking on that theme in the view legend. The geologic points data has
been classified using Unique Value applied to the Symboltype field.
Click on the Advanced Options button on the Legend Editor dialog box and
assign Av_rh_rule (Right-hand Rule rotation) as the rotation field. The
symbols will now be correctly rotated in the view.
- Save the model.
Explore the characteristics of the data used for this example. Study the
relationship of Strike and Dip in sedimentary areas. Review the
orientation of jointing and foliation in old metamorphic terrain. Note
the relationships between point and polygon data.
Strike and Dip
symbols are most numerous in areas where sedimentary rocks, joints,
and foliation are mapped in metamorphic and igneous terrain, and
fluvial features are mapped in young river sediments. Look for breccia
pipes and mine adits. Check out the fluvial directions observed in
young Rio Grande Trough sediments in the northwest portion of the map.
The sample data set's prebuilt legend has been classified, and the appropriate symbols are associated with each data type. To use other geologic point data, thematically symbolize each individual data type by using the Legend Editor to classify the data set using Unique Value and the field in the data set that identifies the type of data. Assign the appropriate symbol from the geology.avp palette.
ArcView GIS Trick
To make a custom legend load with an ArcView GIS theme, use the Legend Editor to save the legend with exactly the same name as the name for the theme's shapefile and store the legend file in the same directory as the shapefile. For example, the shapefile for the exercises in this article is called PLACPNT1.SHP, and its custom legend is PLACPNT1.AVL. When ArcView GIS loads the theme, it will automatically apply a legend with the same name to the theme and load any necessary standard palettes.
Many digitized geologic maps contain several Left-hand points mixed with Right-hand points because the data was recorded with the wrong orientation in the field or was improperly digitized. Data that has a mix of Right-hand and Left-hand points can be handled in one of two ways. If there are just a few entries with Left-hand data, the simplest solution is to correct individual fields in the table by editing them.
If a substantial number of records contains Left-hand data, a query can be used to correct the data. Create a new field in the point data table to flag which type of data is in each record. Choose Theme Properties from the menu and create a query to separate the two orientation types. Duplicate the point theme, symbolize each set, and apply Av_rh_rule to the Right-hand points and Av_lh_rule to the Left-hand points.
To learn more about classifying legends and building queries, see Getting to Know ArcView GIS from Esri Press, which is available from the GIS Store at the Esri Web site.
Summary
This procedure will correctly orient geologic symbols in ArcView GIS 3.1 and 3.2 by joining adjusted Azimuth and Bearing values to point data that may have been incorrectly collected or digitized. The next step might be to explore the USGS EROS Web site and locate other digital geologic mapping data and apply the AZCONV table to attributed point data to properly orient the geologic symbols.
The author thanks the staff of the New Mexico Bureau of Mines and Mineral Resources for graciously providing the digital point and polygon data for this exercise. Please note that this data is preliminary and has not been field checked.
