7.5 minute Digital Elevation Models

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Metadata:


Identification_Information:

Citation:

Citation_Information:

Originator: U.S. Geological Survey

Publication_Date: 19790701

Title: 7.5 minute Digital Elevation Models

Geospatial_Data_Presentation_Form: map

Publication_Information:

Publication_Place: Reston, VA

Publisher: U. S. Geological Survey

Description:

Abstract:

Digital Elevation Model (DEM) is the terminology adopted by
the USGS to describe terrain elevation data sets in a digital
raster form. The standard DEM consists of a regular array of
elevations cast on a designated coordinate projection system.
The DEM data are stored as a series of profiles in which the
spacing of the elevations along and between each profile is in
regular whole number intervals. The normal orientation of
data is by columns and rows. Each column contains a series of
elevations ordered from south to north with the order of the
columns from west to east. The DEM is formatted as one
ASCII header record (A- record), followed by a series of profile
records (B- records) each of which include a short B-record
header followed by a series of ASCII integer elevations per each
profile. The last physical record of the DEM is an accuracy
record (C-record).
7.5-minute DEM (30- by 30-m data spacing, cast on Universal
Transverse Mercator (UTM) projection). Provides coverage in
7.5- by 7.5-minute blocks. Each product provides the same
coverage as a standard USGS 7.5-minute quadrangle without
over edge. Coverage is for the Contiguous United States,
Hawaii, and Puerto Rico.
Purpose:

DEM's can be used as source data for digital orthophotos, and, as
layers in geographic information systems, for earth science
analysis. DEM's can also serve as tools for volumetric analysis, for
site location of towers, or for drainage basin delineation. These
data were collected as part of the National Mapping Program.
Supplemental_Information:

7.5-minute DEMs have rows and columns which vary in length
and are staggered. The UTM bounding coordinates form a
quadrilateral (no two sides are parallel to each other), rather
than a rectangle. The user will need to pad out the uneven
rows and columns with blanks or flagged data values, if a
rectangle is required for the user's application. Some software
vendors have incorporated this function into their software
for input of standard formatted USGS DEMs.
Time_Period_of_Content:

Time_Period_Information:

Range_of_Dates/Times:

Beginning_Date: 19790701

Ending_Date: Present

Currentness_Reference: ground condition

Status:

Progress: In work

Maintenance_and_Update_Frequency: Irregular

Spatial_Domain:

Bounding_Coordinates:

West_Bounding_Coordinate: -124.7333

East_Bounding_Coordinate: -067.9500

North_Bounding_Coordinate: 49.3833

South_Bounding_Coordinate: 24.5333

Keywords:

Theme:

Theme_Keyword_Thesaurus: none

Theme_Keyword: DEM

Theme_Keyword: digital elevation model

Theme_Keyword: digital terrain model

Theme_Keyword: hypsography

Theme_Keyword: altitude

Theme_Keyword: height

Theme_Keyword: contour line

Theme_Keyword: digital contours

Place:

Place_Keyword_Thesaurus:

U.S. Department of Commerce, 1977, Countries, dependencies,
areas of special sovereignty, and their principal administrative divisions
(Federal Information Processing Standard 10-3):Washington, D.C.,
National Institute of Standards and Technology.
Place_Keyword: US

Place_Keyword: CA

Place_Keyword: MX

Place:
Place_Keyword_Thesaurus:

U.S. Department of Commerce, 1987, Codes for the identification
of the States, the District of Columbia and the outlying areas of
The United States, and associated areas
(Federal Information Processing Standard 5-2): Washington, D. C.,
National Institute of Standards and Technology.
Place_Keyword: FIPS code of State or Province

Place:
Place_Keyword_Thesaurus:

U.S. Department of Commerce,1990, Counties and equivalent
entities of The United States, its possessions, and associated areas
(Federal Information Processing Standard 6-4): Washington, D.C.
National Institute of Standards and Technology.
Place_Keyword: FIPS code for county or counties.

Access_Constraints: None

Use_Constraints:

None. Acknowledgement of the U.S. Geological Survey would be
appreciated in products derived from these data.


Data_Quality_Information:

Attribute_Accuracy:

Attribute_Accuracy_Report:

The accuracy of a DEM is dependent upon the level of
detail of the source and the grid spacing used to sample
that source. The primary limiting factor for the level of
detail of the source is the scale of the source materials.
The proper selection of grid spacing determines the level
of content thatmay be extracted from a given source
during digitization.
Logical_Consistency_Report:

The fidelity of the relationships encoded in the data
structure of the DEM are automatically verified using
a USGS software program upon completion of the data
production cycle. The test verifies full compliance to the
DEM specification.
Completeness_Report:

The DEM is visually inspected for completeness on a DEM view
and edit system for the purpose of performing a final quality
control and if necessary edit of the DEM. The physical format
of each digital elevation model is validated for content
completeness and logical consistency during production quality
control and prior to archiving in the National Digital
Cartographic Data Base.
Due to the variable orientation of the quadrilateral in relation
to the Universal Transverse Mercator (UTM) projection grid,
profiles that pass within the bounds of the DEM quadrilateral,
may be void of elevation grid points, and are not represented
in the DEM. This condition occurs infrequently and is always the
first or last profile of the dataset.
Level 2 DEM: Level 2 DEM's may contain void areas due to
interruptions to contours in the source graphic or DLG. Void
area elevation grid posts are assigned the value of -32,767. In
addition, suspect elevation areas may exist in the DEM but are
not specifically identified. Suspect areas can be located on the
source graphic as a "disturbed surface, " symbolized by
contours overprinted with photorevised or other surface
patterns.
Positional_Accuracy:

Horizontal_Positional_Accuracy:

Horizontal_Positional_Accuracy_Report:

The horizontal accuracy of the DEM is expressed
as an estimated root mean square error (RMSE).
The estimate of the RMSE is based upon
horizontal accuracy tests of the DEM source
materials which are selected as equal to or less
than intended horizontal RMSE error of the DEM.
The testing of horizontal accuracy of the source
materials isaccomplished by comparing the
planimetric (X and Y) coordinates of well-defined
ground points with the coordinates of the same
points as determined from a source of higher
accuracy.

Vertical_Positional_Accuracy:

Vertical_Positional_Accuracy_Report:

The vertical RMSE statistic is used to describe the
vertical accuracy of a DEM, encompassing both
random and systematic errors introduced during
production of the data. The RMSE is encoded in
element number 5 of record C of the DEM.
Accuracy is computed by a comparison of linear
interpolated elevations in the DEM with
corresponding known elevations. Test points are
well distributed, representative of the terrain, and
have true elevations with accuracies well within
the DEM accuracy criteria. Acceptable test points
include, in order of preference: field control,
aerotriangulated test points, spot elevations, or
points on contours from existing source maps
with appropriate contour interval. A minimum of
28 test points per DEM is required to compute the
RMSE, which is composed of a single test using 20

interior points and 8 edge points. Edge points are
those which are located along, at, or near the
quadrangle neatlines and are deemed by the
editor to be useful to evaluating the accuracy of
the edge of the DEM. Collection of test point data
and comparison of the DEM with the quadrangle
hypsography are conducted by the quality control
units within the USGS.
There are three types of DEM vertical errors;
blunder, systematic and random. These errors
are reduced in magnitude by editing but cannot be
completely eliminated. Blunder errors are those
errors of major proportions and are easily
identified and removed during interactive editing.
Systematic errors are those errors that follow
some fixed pattern and are introduced by data
collection procedures and systems. These error
artifacts include: vertical elevation shifts,
misinterpretation of terrain surface due to trees,
buildings and shadows, and fictitious ridges, tops,
benches or striations. Random errors result from
unknown or accidental causes.
DEM's are edited to correctly depict elevation
surfaces that correspond to water bodies of
specified size.
Level 1 DEM: A RMSE of 7-meters or less is the
desired accuracy standard. A RMSE of 15-meters
is the maximum permitted. A 7.5-minute DEM at
this level has an absolute elevation error
tolerance of 50 meters (approximately three
times the 15-meter RMSE) for blunder errors for
any grid node when compared to the true
elevation. Any array of points in the DEM can not
encompass more than 49 contiguous elevations
in error by more than 21 meters (three times the
7-meter RMSE). Systematic errors that are
within stated accuracy standards are tolerated.
Level 2 DEM: A vertical RMSE of one-half of the
contour interval, determined by the source map,
is the maximum permitted. Systematic errors
may not exceed one contour interval,
determined by the source map, is the maximum
permitted. Systematic errors may not exceed
one contour interval specified by the source
graphic. Level 2 DEMs have been processed or
smoothed for consistency and edited to remove
identifiable systematic errors.
Lineage:

Source_Information:

Source_Citation:

Citation_Information:

Originator: U.S. Geological Survey

Publication_Date: Unknown

Title: digital contour lines

Geospatial_Data_Presentation_Form: map

Publication_Information:

Publication_Place: Reston, VA

Publisher: U.S. Geological Survey

Type_of_Source_Media: magnetic tape

Source_Time_Period_of_Content:

Time_Period_Information:

Range_of_Dates/Times:

Beginning_Date: 19790701

Ending_Date: Present

Source_Currentness_Reference: ground condition

Source_Citation_Abbreviation: CONTOUR1

Source_Contribution:

hypsographic vector information which is interpolated to regular
grid posts to form DEM grids in 30- by 30- meter UTM data spacing
within the 7.5 minute DEM bounds.
Source_Information:

Source_Citation:

Citation_Information:

Originator: U.S. Geological Survey

Publication_Date: Unknown

Title: photo ID number

Geospatial_Data_Presentation_Form: remote-sensing image

Publication_Information:

Publication_Place: Reston, VA

Publisher: U.S. Geological Survey

Type_of_Source_Media: transparency

Source_Time_Period_of_Content:

Time_Period_Information:

Range_of_Dates/Times:

Beginning_Date: Unknown

Ending_Date: Present

Source_Currentness_Reference: ground condition

Source_Citation_Abbreviation: PHOTO1

Source_Contribution: elevation values

Source_Information:

Source_Citation:

Citation_Information:

Originator: U.S. Geological Survey

Publication_Date: Unknown

Title: project control

Geospatial_Data_Presentation_Form: map

Publication_Information:

Publication_Place: Reston, VA

Publisher: U.S. Geological Survey

Type_of_Source_Media: magnetic tape

Source_Time_Period_of_Content:

Time_Period_Information:

Range_of_Dates/Times:

Beginning_Date: Unknown

Ending_Date: Present

Source_Currentness_Reference: ground condition

Source_Citation_Abbreviation: CONTROL1

Source_Contribution: ground control points

Process_Step:

Process_Description:

The production procedures, instrumentation,
hardware and software used in the collection of
standard U. S. Geological Survey (USGS) Digital
Elevation Models (DEM's) vary depending on
systems used at the contractor, cooperator or
National Mapping Division (NMD) production sites.
This process step describes, in general, the
process used in the production of standard USGS
DEM datasets.
Level 1 DEM: Level 1 DEM's are acquired
photogrammetrically by manual profiling or image
correlation techniques from National Aerial
Photography Program (NAPP) or equivalent
source photographs. Level 1 30-minute DEM's
may be derived or resampled from level 1 7.5-
minute DEM's.
Level 2 DEM: Level 2 DEM's are produced by
converting 1:24,000-scale and 1:100,000-scale
hypsography digital line graph (DLG) data to DEM
format or the DEM's are generated from vector
data derived from scanned raster files of USGS
1:24.000-scale or 1:100,000-scale map series
contour separates.
Level 3 DEM: Level 3 DEM's are created from DLG
data that has been vertically integrated with all
categories of hypsography, hydrography, ridge
line, break line, drain files and all vertical and
horizontal control networks. The production of
level 3 DEMs requires a system of logic
incorporated into the software interpolation
algorithms that clearly differentiates and
correctly interpolates between the various types
of terrain, data densities and data distribution.
Water body editing: DEM surface areas
corresponding to water bodies are flattened and
assigned map specified or estimated surface
elevations. Water body areas are defined as
ponds, lakes, and reservoirs that exceed 0.5
inches at map scale and double line drainage that
exceeds 0.25 inches at map scale. Water body
shorelines are derived either from a hypsographic
DLG or by interactive delineation from 1:24,000-
scale or 1:100,000-scale USGS map series.
Edge matching: DEM datasets within a project area
(consisting of a number of adjacent files) are edge
matched to assure terrain surface continuity
between files. Edge matching is the process of
correcting adjacent elevation values along common edges.
The objective of edge matching is to create more accurate
terrain representations by correcting the alignment of
ridges and drains, and overall topographic shaping within
an approximately 25-30 row or column grid post
zone on both edges.
Quality control: DEM's are viewed on interactive
editing systems to identify and correct blunder
and systematic errors. DEM's are verified for
physical format and logical consistency at the
production centers and before archiving in the
National Digital Cartographic Data Base (NDCDB)
utilizing the Digital Elevation Model Verification
System (DVS) software.
Source_Used_Citation_Abbreviation: CONTOUR1, PHOTO1, CONTROL1

Process_Date: Unknown



Spatial_Data_Organization_Information:

Direct_Spatial_Reference_Method: raster

Raster_Object_Information:

Raster_Object_Type: grid cell



Spatial_Reference_Information:

Horizontal_Coordinate_System_Definition:

Planar:

Grid_Coordinate_System:

Grid_Coordinate_System_Name: Universal Transverse Mercator

Universal_Transverse_Mercator:

UTM_Zone_Number: 10-19

Transverse_Mercator:

Scale_Factor_at_Central_Meridian: .9996

Longitude_of_Central_Meridian: -123 00 00

Latitude_of_Projection_Origin: 0.0

False_Easting: 500000

False_Northing: 0.0

Planar_Coordinate_Information:

Planar_Coordinate_Encoding_Method: row and column

Coordinate_Representation:

Abscissa_Resolution: 30

Ordinate_Resolution: 30

Planar_Distance_Units: meters

Geodetic_Model:

Horizontal_Datum_Name: North American Datum 1927

Ellipsoid_Name: Clark 1866

Semi-major_Axis: 6378206.4

Denominator_of_Flattening_Ratio: 294.9787

Vertical_Coordinate_System_Definition:

Altitude_System_Definition:

Altitude_Datum_Name: National Geodetic Vertical Datum of 1929

Altitude_Resolution: 1

Altitude_Distance_Units: feet or meters

Altitude_Encoding_Method:
explicit elevation coordinate included with horizontal coordinates



Entity_and_Attribute_Information:

Overview_Description:

Entity_and_Attribute_Overview:

The digital elevation model is composed of a 6-
character integer raster representing a gridded form of
a topographic map hypsography overlay. Each raster
entity contains a 6-character integer value between
-32,767 to 32,768.
Entity_and_Attribute_Detail_Citation:

U.S.Department of the Interior, U.S. Geological Survey,
Digital Elevation Models -- Data Users Guide; Reston, VA,
A hypertext extract is available at:
URL: <http://edc.usgs.gov/glis/hyper/guide/usgs_dem>
Softcopies in ASCII and WordPerfect format are available at:
URL: <ftp://mapping.usgs.gov/pub/ti/DEM/>
U.S.Department of the Interior, U.S. Geological Survey, 1992,
Standards for digital elevation models: Reston, VA,
Softcopies in ASCII and WordPerfect format are available at:
URL: <ftp://mapping.usgs.gov/pub/ti/DEM/>


Distribution_Information:

Distributor:

Contact_Information:

Contact_Organization_Primary:

Contact_Organization: Earth Science Information Center, U.S. Geological Survey

Contact_Address:

Address_Type: mailing address

Address: 507 National Center

City: Reston

State_or_Province: Virginia

Postal_Code: 20192

Contact_Voice_Telephone: 1-888-ASK-USGS

Hours_of_Service: 0800-1600

Contact_Instructions:

In addition to the address above there are other ESIC offices
throughout the country. A full list of these offices is at:
URL: <http://mapping.usgs.gov/esic/esic_index.html>
Resource_Description: 7.5-minute digital elevation models

Distribution_Liability:

Although these data have been processed successfully on a
computer system at the U.S. Geological Survey, no warranty
expressed or implied is made by the Geological Survey
regarding the utility of the data on any other system, nor shall
the act of distribution constitute any such warranty. USGS will
warrant the delivery of this product in computer-readable
format. and will offer appropriate adjustment of credit when
the product is determined unreadable by correctly adjusted
computer input peripherals, or when the physical medium is
delivered in damaged condition. Requests for adjustments of
credit must be made within 90 days from the date of this
shipment from the ordering site.
Standard_Order_Process:

Digital_Form:

Digital_Transfer_Information:

Format_Name: DEM

Format_Information_Content:

USGS standard DEM: The standard USGS
DEM can be described as an ASCII formatted
elevation file preceded by a metadata
header file which consists of one 1024 byte
ACSII record.
Transfer_Size: 1

Digital_Transfer_Option:

Offline_Option:

Offline_Media: CD-ROM

Recording_Capacity:

Recording_Density: 680000

Recording_Density_Units: megabytes

Recording_Format: ISO 9660

Compatibility_Information:

This CD-ROM can be used with all computer operating systems
that support CD-ROM as a logical storage device. All text
files on the disc are in ASCII format. Data files are in
ASCII or binary format.
Offline_Option:

Offline_Media: Cartridge

Recording_Capacity:

Recording_Density: 3480

Recording_Density_Units: megabytes

Recording_Format:

ASCII; available unlabelled or with ANSI-standard labels;
available block sizes are multiples of 1024 characters. To
ensure compatibility, a maximum blocking factor of 31744 is
recommended. For efficiency, blocking factors less than
16,000 are discouraged.
Offline_Option:

Offline_Media: Cassette

Recording_Capacity:

Recording_Density: 8-mm High (4.5Gb)

Recording_Density_Units: gigabytes

Recording_Format:

ASCII; available unlabelled or with ANSI-standard labels;
available block sizes are multiples of 1024 characters. To
ensure compatibility, a maximum blocking factor of 31744 is
recommended. For efficiency, blocking factors less than
16,000 are discouraged.
Offline_Option:

Offline_Media: Cassette

Recording_Capacity:

Recording_Density: 8-mm Low (2.3Gb)

Recording_Density_Units: gigabytes

Recording_Format:

ASCII; available unlabelled or with ANSI-standard labels;
available block sizes are multiples of 1024 characters. To
ensure compatibility, a maximum blocking factor of 31744 is
recommended. For efficiency, blocking factors less than
16,000 are discouraged.
Fees:

The online copy of the data set (when available electronically) may
be accessed without charge. For cartridge, cassette, and CD-ROM the
costs are available at:
URL: <http://edc.usgs.gov/glis/hyper/order_info/prices#7_MIN_DEM>
Ordering_Instructions:

Dataset searching and ordering capabilities are available through the
Global Land Information System (GLIS) at:
URL: <http://edc.usgs.gov/webglis>

Metadata: Reference Information

Metadata_Reference_Information:

Metadata_Date: 199501

Metadata_Contact:

Contact_Information:

Contact_Organization_Primary:

Contact_Organization: U.S. Geological Survey

Contact_Address:

Address_Type: mailing address

Address: 508 National Center

City: Reston

State_or_Province: VA

Postal_Code: 20192

Contact_Voice_Telephone: 703 648 4543

Metadata_Standard_Name: Content Standards for Digital Geospatial Metadata

Metadata_Standard_Version: FGDC-STD-001-1998

Metadata_Security_Information:

Metadata_Security_Classification_System: None

Metadata_Security_Classification: Unclassified

Metadata_Security_Handling_Description: None

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