Isostatic gravity map of the Los Angeles 30 x 60 minute quadrangle, California

Frequently anticipated questions:

• What does this data set describe?
  1. How might this data set be cited?
  2. What geographic area does the data set cover?
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
  5. What is the general form of this data set?
  6. How does the data set represent geographic features?
  7. How does the data set describe geographic features?
• Who produced the data set?
  1. Who are the originators of the data set?
  2. Who also contributed to the data set?
  3. To whom should users address questions about the data?
• Why was the data set created?
• How was the data set created?
  1. From what previous works were the data drawn?
  2. How were the data generated, processed, and modified?
  3. What similar or related data should the user be aware of?
• How reliable are the data; what problems remain in the data set?
  1. How well have the observations been checked?
  2. How accurate are the geographic locations?
  3. How accurate are the heights or depths?
  4. Where are the gaps in the data? What is missing?
  5. How consistent are the relationships among the data, including topology?
• How can someone get a copy of the data set?
  1. Are there legal restrictions on access or use of the data?
  2. Who distributes the data?
  3. What's the catalog number I need to order this data set?
  4. What legal disclaimers am I supposed to read?
  5. How can I download or order the data?
• Who wrote the metadata?

What does this data set describe?

1. How might this data set be cited?
   Wooley, R.J., Yerkes, R.F., Langenheim, V.E., and Chuang, F.C., 2003, Isostatic gravity map of the Los Angeles 30 x 60 minute quadrangle, California: U.S. Geological Survey Open-File Report 03-269, U.S. Geological Survey, U.S. Geological Survey, Menlo Park, CA.

   Online Links:

   • http://pubs.usgs.gov/of/2003/0269/

2. What geographic area does the data set cover?

   West_Bounding_Coordinate: -119.0
   East_Bounding_Coordinate: -118.0
   North_Bounding_Coordinate: 34.5
   South_Bounding_Coordinate: 34.0
   

3. What does it look like?
4. Does the data set describe conditions during a particular time period?

   Beginning_Date: 1994

   Ending_Date: 2001

   Currentness_Reference:

   Data collection period

5. What is the general form of this data set?

   Geospatial_Data_Presentation_Form: map
   

6. How does the data set represent geographic features?
   1. How are geographic features stored in the data set?
      This is a Point data set. It contains the following vector data types (SDTS terminology):
      • Entity point (3914)
   2. What coordinate system is used to represent geographic features?
      Horizontal positions are specified in geographic coordinates, that is, latitude and longitude. Latitudes are given to the nearest 0.00017. Longitudes are given to the nearest 0.00017. Latitude and longitude values are specified in decimal degrees. The horizontal datum used is North American Datum of 1927.
      The ellipsoid used is Geodetic Reference System 1967.
      The semi-major axis of the ellipsoid used is 6378160.
      The flattening of the ellipsoid used is 1/298.25.
      
7. How does the data set describe geographic features?

   station

   Point at which gravity anomaly was measured

   STAT

   Station Name--An alphanumeric combination of up to 8 characters used for station identification textual identifier without scientific significance

   LaD

   Degree Latitude North

   Range of values
   Minimum:	34
   Maximum:	34
   Units:	degrees
   Resolution:	1

   LaM

   Decimal Minute

   Range of values
   Minimum:	0.00
   Maximum:	60.00
   Units:	minutes
   Resolution:	0.01

   LoD

   Degree Longitude West

   Range of values
   Minimum:	117
   Maximum:	118
   Units:	degrees
   Resolution:	1

   LoM

   Decimal Minute

   Range of values
   Minimum:	0.00
   Maximum:	60.00
   Units:	minutes
   Resolution:	0.01

   ELEV

   Elevation in feet

   Range of values
   Minimum:	16.0
   Maximum:	7111.0
   Units:	feet
   Resolution:	0.1

   acc

   accuracy indicator?

   Value	Definition
   (no value)
   A
   b
   m
   t
   U

   OG

   Observed gravity, in mGal

   Range of values
   Minimum:	979151.31
   Maximum:	979650.10
   Units:	mGal
   Resolution:	0.01

   FAA

   Free-air anomaly, in mGal

   Range of values
   Minimum:	-98.01
   Maximum:	140.77
   Units:	mGal
   Resolution:	0.01

   SBA

   Simple Bouguer anomaly, in mGal

   Range of values
   Minimum:	-121.18
   Maximum:	3.93
   Units:	mGal
   Resolution:	0.01

   ITC

   Inner terrain correction for a density of 2.67 g/cc, in mGal. See Code column for radius of correction.

   Range of values
   Minimum:	0.00
   Maximum:	11.00
   Units:	mGal
   Resolution:	0.01

   TTC

   Total terrain correction from the station to 166.7 km for a density of 2.67 g/cc, in mGal.

   Range of values
   Minimum:	0.00
   Maximum:	31.97
   Units:	mGal
   Resolution:	0.01

   Code

   Letter denoting the zone (radius) of the correction

   Value	Definition
   (no value)
   D	0.59 km
   G	3.52 km
   M	58.8 km

   CBA

   Complete Bouguer anomaly reduced for a density of 2.67 g/cc, in mGal.

   Range of values
   Minimum:	-115.45
   Maximum:	6.83
   Units:	mGal
   Resolution:	0.01

   ISO

   Isostatic residual anomaly,in mGal,assuming an Airy model for isostatic compensation of topographic loads. This model assumes a crustal thickness of 25 km, a topographic load density of 2.67 g/cc, and a density contrast across the base of the model crust of 0.4 g/cc. Isostatic correction would be difference between CBA and ISO values.

   Range of values
   Minimum:	-71.31
   Maximum:	17.10
   Units:	mGal
   Resolution:	0.01

Who produced the data set?

1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
   • R.J. Wooley
   • R.F. Yerkes
   • V.E. Langenheim
   • F.C. Chuang
2. Who also contributed to the data set?

   We would like to thank Doug Morton and the Southern California Areal Mapping Project for providing funding for the creation of this map. We would also like to acknowledge and thank Seth Stiles for help in collecting gravity data within the Los Angeles 30' x 60' quadrangle. The map benefited from reviews by David Ponce (U.S. Geological Survey, Menlo Park, CA).

3. To whom should users address questions about the data?
   Victoria E Langenheim

   U.S. Geological Survey, GEO-WRG-NGM

   Mail Stop 989

   345 Middlefield Rd

   Menlo Park, CA
   

   USA

   650-329-5313 (voice)

   650-329-5133 (FAX)

   zulanger@usgs.gov

Why was the data set created?

This isostatic residual gravity map is part of the Southern California Areal Mapping Project (SCAMP) and is intended to promote further understanding of the geology in the Los Angeles 30 x 60 minute quadrangle, California, by serving as a basis for geophysical interpretations and by supporting both geological mapping and topical (especially earthquake) studies. Local spatial variations in the Earth' s gravity field (after various corrections for elevation, terrain, and deep crustal structure explained below) reflect the lateral variation in density in the mid- to upper crust. Densities often can be related to rock type, and abrupt spatial changes in density commonly mark lithologic boundaries.

How was the data set created?

1. From what previous works were the data drawn?

   Hanna and others (1975) (source 1 of 1)

   Hanna, W.F., Rietman, J.D., and Biehler, S.H., 1975, Bouguer gravity map of California, Los Angeles sheet: California Division of Mines and Geology.

   Type_of_Source_Media: paper
   Source_Scale_Denominator: 250000
   

2. How were the data generated, processed, and modified?

   Date: 1994 (process 1 of 2)

   Gravity data in the Los Angeles 1:100,000-scale quadrangle and vicinity include 428 gravity stations obtained by the U.S. Geological Survey from 1994 to June, 2001; 2536 stations from a previously published Bouguer Gravity Map of California--Los Angeles sheet compiled by W.F. Hanna and others, (1974); and 950 gravity stations obtained from the Defense Mapping Agency (written communication, 1994). Gravity stations collected by the U.S. Geological Survey for the Los Angeles Regional Seismic Experiment line II (LARSE II) are also included. More detailed information on recent U.S. Geological Survey data collection in the Los Angeles area is contained in Wooley and Langenheim (2001). The datum of observed gravity for this map is the International Gravity Standardization Net of 1971 (IGSN 71) as described by Morelli (1974); the reference ellipsoid used is the Geodetic Reference System 1967 (GRS67; International Association of Geodesy and Geophysics, 1971).
   The observed gravity data were reduced to free-air anomalies using standard formulas (e.g. Telford and others, 1976). Bouguer, curvature, and terrain corrections to a radial distance of 166.7 km) were applied to the free-air anomaly at each station to determine the complete Bouguer anomalies at a standard reduction density of 2.67 g/ cm3 (Plouff, 1977). An isostatic correction was then applied to remove the long-wavelength effect of deep crustal and/or upper mantle masses that isostatically support regional topography. The isostatic correction assumes an AiryHeiskanen model (Heiskanen and Vening-Meinesz, 1958) of isostatic compensation; compensation is achieved by varying the depth of the model crust-mantle interface, using the following parameters: a sea-level crustal thickness of 25 km, a crust-mantle density contrast of 0.40 g/cm3, and a crustal density of 2.67 g/cm3 for the topographic load. These parameters were used because (1) they produce a model crustal geometry that agrees with seismically determined values of crustal thickness for central California, (2) they are consistent with model parameters used for isostatic corrections computed for the rest of California (Roberts and others, 1990), and (3) changing the model parameters does not significantly affect the resulting isostatic anomaly (Jachens and Griscom, 1985). The computer program ISOCOMP (Jachens and Roberts, 1981) directly calculates the attraction of an Airy-Heiskanen root by summing the attraction of individual mass prisms making up the root and thus calculating the isostatic correction; the resulting isostatic residual gravity values should reflect lateral variations of density within the mid- to upper crust.

   Date: 15-Sep-2003 (process 2 of 2)

   Creation of original metadata record Person who carried out this activity:
   

   Victoria E Langenheim

   U.S. Geological Survey, GEO-WRG-NGM

   Mail Stop 989

   345 Middlefield Rd

   Menlo Park, CA
   

   USA

   650-329-5313 (voice)

   650-329-5133 (FAX)

   zulanger@usgs.gov

3. What similar or related data should the user be aware of?

How reliable are the data; what problems remain in the data set?

1. How well have the observations been checked?
   Observed gravity data are accurate to 0.05 mGal; reduced anomalies are accurate to 1-2 mGal. Data are more accurate in the valleys than in the mountains because of uncertainties in elevation and in the terrain correction.
2. How accurate are the geographic locations?
   Accuracy of the point data is roughly 40 m, as well as the data can be located on a 1:24,000-scale topographic map
3. How accurate are the heights or depths?
   Accuracy of the point data vary; most of the elevations are accurate to 10 feet or less. In some of the more mountainous areas, the accuracy may fall to 20 to 40 feet.
4. Where are the gaps in the data? What is missing?
   Spacing of observation points in the study area is dense but not regular; generally the density of observation points is lowest in the northeast corner of the quadrangle.
5. How consistent are the relationships among the observations, including topology?
   The main sources of error are inaccurate terrain corrections and/or inaccurate elevations. Errors associated with terrain corrections may be 5 to 10 percent of the value of the total terrain correction. The average error based on the average terrain correction (2.48 mGal) is thus about 0.2 mGal, but in the most rugged areas of the San Gabriel Mountains, the individual errors may be as large as 3.2 mGal. Errors resulting from elevation uncertainties are probably less than 0.5 mGal for most of the data because the majority of the stations are at or near benchmarks and spot and surveyed elevations, which are accurate to about 0.2 to 3 m. Measurements for which elevations were controlled by contour interpolation are expected to have errors of up to 1.2 mGal. The significance of errors resulting from elevation is greater in mountainous areas, where elevations vary over a relatively short distance, than in the flatter valleys and deserts. In general, the total uncertainties for the data shown on the map are estimated to be less than 2 mGal (or one contour interval), although in many areas the data are considerably more accurate.

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?

Access_Constraints: none
Use_Constraints: none

1. Who distributes the data set? (Distributor 1 of 1)
   
   USGS Information Services

   Box 25286 Denver Federal Center

   Denver, CO
   

   USA

   1-888-ASK-USGS (voice)

   303-202-4693 (FAX)

   ask@usgs.gov
2. What's the catalog number I need to order this data set? USGS Open-File Report 03-269
3. What legal disclaimers am I supposed to read?

   This report is preliminary and has not been reviewed for conformity with the U.S. Geologcial Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

4. How can I download or order the data?
   • Availability in digital form:

     Data format:

     Gravity observation data in format ASCII Strict-format ASCII text as follows; here S = start column E = end column (inclusive) T = data type (C character, N numeric) W = width P = "precision", digits to right of decimal Label S E T W P STAT 1 8 C 8 LaD 10 11 N 2 0 LaM 13 17 N 5 2 LoD 19 21 N 3 0 LoM 23 27 N 5 2 ELEV 29 35 N 7 1 OG 37 45 N 9 2 47 47 C 1 FAA 52 58 N 7 2 SBA 60 66 N 7 2 ITC 68 73 N 6 2 TTC 75 80 N 6 2 Code 82 82 C 1 CBA 84 90 N 7 2 ISO 92 98 N 7 2 Size: 0.3691

     Network links:

     http://pubs.usgs.gov/of/2003/0269/la-grav.dat

   • Cost to order the data: none

Who wrote the metadata?

Dates:

Last modified: 05-Feb-2016

Metadata author:

Peter N Schweitzer

USGS Midwest Area

Collection manager, USGS Geoscience Data Clearinghouse, http://geo-nsdi.er.usgs.gov/

Mail Stop 954

12201 Sunrise Valley Dr

Reston, VA

USA

703-648-6533 (voice)

703-648-6252 (FAX)

pschweitzer@usgs.gov

Metadata standard:

Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)