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 includes 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).
The 7.5-minute DEM's provided through GRANIT are standard USGS
7.5-minute files reprocessed to conform to GRANIT standards, as
described below. The elevation values are reported in feet,
and are spaced at 30-meter intervals along and between
each profile.
Purpose:
DEM's can be used as source data for generating digital orthophotos,
and/or as layers in geographic information systems for earth
science analysis (e.g. contour, slope, hillshade
derivatives).
Supplemental_Information:
Data distribution tile: 7.5-minute quadrangle.
Note that the DEMs are in an ASCII file format, e.g. standard
USGS DEM format, and require conversion to be used within
standard GIS packages. Most packages will provide a utility
to accomplish this conversion.
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: -72.125
East_Bounding_Coordinate: -72.000
North_Bounding_Coordinate: 43.750
South_Bounding_Coordinate: 43.625
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: None
Place_Keyword: United States
Place_Keyword: Northeast
Place_Keyword: New England
Place_Keyword: New Hampshire
Place_Keyword: CANAAN, NH
Access_Constraints: None
Use_Constraints: Not for legal use
Point_of_Contact:
Contact_Information:
Contact_Organization_Primary:
Contact_Organization: Complex Systems Research Center
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 that may 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. (Note, however, that GRANIT has
re-processed the data since retrieval from USGS, as
referenced in the Process Description section.)
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.
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 is accomplished 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:
US Geological Survey or another mapping agency in
cooperation with USGS
Hysographic 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.
The source DEM files were reprocessed to
conform to GRANIT database standards. Key
elements of the processing included:
- reprojecting the spatial data from
UTM meters (Zone 18/19), NAD27 to
NH State Plane feet, Zone 4676,
NAD83.
- standardizing all Z-values to feet
The specific steps applied to each data file
included the following:
1. Data for each quad was retrieved from the USGS web site
in STDS format as a tarred, compressed (gzipped) file.
(<http://edcwww.cr.usgs.gov/doc/edchome/ndcdb/7_min_dem/states/NH.html>)
2. The single files were uncompressed and untarred into
quad-specific directories.
(usage: gunzip -c 30.2.nnnn.tar.gz | tar -xpvf - )
3. In Imagine:
a) Load the *IDEN.DDF data to create the *.img file
b) Convert the *.img file to an Arc/Info compatible GRID
4. In Arc/Info:
a) Run STDSINFO and record relevant parameters
b) Run PROJECTDEFINE on the grid
c) Check cell size of each grid, and standardize to
to meters if necessary
d) Reproject the GRID to NH State Plane feet, NAD83
e) Convert Z-units from meters to feet, if necessary
f) Convert all edge values from their default value
of -32767 to "no data"
g) Run LATTICEDEM to convert GRID back to ASCII DEM
format
Source_Used_Citation_Abbreviation: DEM-1
Process_Date: 19990301
Source_Produced_Citation_Abbreviation: Version 1
Process_Contact:
Contact_Information:
Contact_Person_Primary:
Contact_Organization: Complex Systems Research Center
Contact_Person: GRANIT Database Manager
Contact_Position: GRANIT Database Manager
Contact_Address:
Address_Type: mailing and physical address
Address: Morse Hall, University of New Hampshire
City: Durham
State_or_Province: NH
Postal_Code: 03824
Country: US
Contact_Voice_Telephone: 603-862-1792
Contact_Facsimile_Telephone: 603-862-0188
Contact_Electronic_Mail_Address: granit@unh.edu
Hours_of_Service: 8:30AM-5PM, EST
Process_Step:
Process_Description:
All preliminary process steps were performed
by USGS in the development of the source DEM's,
as described below.
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.
Grid_Coordinate_System_Name: State Plane Coordinate System 1983
State_Plane_Coordinate_System:
SPCS_Zone_Identifier: New Hampshire
Transverse_Mercator:
Scale_Factor_at_Central_Meridian: 0.999967
Longitude_of_Central_Meridian: -71.666667
Latitude_of_Projection_Origin: 42.500000
False_Easting: 984249.999900
False_Northing: 0.000000
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 of 1983
Ellipsoid_Name: GRS 80
Semi-major_Axis: 20925604.4720406
Denominator_of_Flattening_Ratio: 298.26
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 hysography 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,
Contact_Organization: Complex Systems Research Center
Contact_Person: GRANIT Database Manager
Contact_Position: GRANIT Database Manager
Contact_Address:
Address_Type: mailing and physical address
Address: Morse Hall, University of New Hampshire
City: Durham
State_or_Province: NH
Postal_Code: 03824
Country: US
Contact_Voice_Telephone: 603-862-1792
Contact_Facsimile_Telephone: 603-862-0188
Contact_Electronic_Mail_Address: granit@unh.edu
Hours_of_Service: 8:30AM-5PM, EST
Resource_Description: DEM93 - 7.5-Minute Digital Elevation Model for Quad 93
Distribution_Liability:
Digital data in NH GRANIT represent the efforts of the
contributing agencies to record information from the cited
source materials. Complex Systems Research Center, under
contract to the NH Office of State Planning, and in
consultation with cooperating agencies, maintains a
continuing program to identify and correct errors in these
data. OSP, CSRC, and the cooperating agencies make no claim
as to the validity or reliability or to any implied uses of
these data.
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.
