Chief Directorate:
National Geo –spatial Information
Standard for Digital Elevation Model Data
Document Reference:
Version:
QLAS.SD.3_v1
v1
This document is controlled in terms of the Quality Management System and may not be
edited, altered or printed without permission of the Division: Quality Assurance.
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SECTION A
Preliminary Informative Elements
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A1
Document Control
Version and Amendment Schedule
Version No.
Version Date
Compiler
Description of Amendments
Draft 1
13 September 2007
Document Created
Draft 2
25 August 2011
Draft 3
02 November 2011
v1
09 November 2011
Standards
Committee
Raoul Duesimi
Sissiel Kay
Raoul Duesimi
Sissiel Kay
Derek Clarke
Professional Team
Sissiel Kay
Applied public review amendments
Applied senior management review
amendments
Versioning
Approval and Control Schedule
Version Approved By
No.
Designation
v1
Chief Director
Derek Clarke
QLAS.SD.3_v1_2011.11.09
Signature
Date Approved
Copy Status
09 November 2011
Master Copy
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A2
Table of Contents
A1
A3
A4
A5
A6
B1
B2
B3
C1
Document Control ................................................................................................................... 3
Foreword ................................................................................................................................. 5
Terms and Definitions ............................................................................................................. 6
Symbols and Abbreviations .................................................................................................... 7
Introduction ............................................................................................................................. 8
Digital Elevation Data ........................................................................................................... 11
Digital Elevation Model ......................................................................................................... 12
Elevation Data Generation ................................................................................................... 14
References ........................................................................................................................... 17
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A3
Foreword
This standard covers the requirements for the production of Digital Elevation Model (DEM) data.
The Chief Directorate: National Geo–spatial Information (CD: NGI) is responsible for the distribution
of Digital Elevation Model data. A national 25m DEM is achieved by computing DEMS from
vectorised contours, or from photogrammetrically sourced data in selected areas. The CD: NGI is
mandated by the Land Survey Act, Act 8 of 1997 to provide mapping and related geo-spatial
information that covers the extent of the Republic of South Africa.
Acknowledgement
This standard was developed by the following members of the Standards Committee:
Raoul Duesimi (Convenor)
Mary Ball
Jacques Havenga
Kevin Scott
Herman Pheiffer (Quality Assurance - Staff Member)
Nazeem Soeker (Quality Assurance - Staff Member)
Sissiel Kay (Quality Assurance - Staff Member)
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A4
Terms and Definitions
Refer to Glossary
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A5
Symbols and Abbreviations
Refer to Glossary
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A6
Introduction
A 6.1 General
Name of the Standard
The standard described in this document shall be known as the Standard for Digital Elevation
Model Data
The shortened name, DEM Standard, may be used.
A 6.2 Scope and Purpose
The Standard for Digital Elevation Model Data comprises all the specifications required for
generating digital elevation model data as required by the CD: NGI and its status is mandatory.
A 6.3 Audience
The primary audience of this document is the CD: NGI and contractors. The secondary audience of
this document are geo-spatial data users.
A 6.4 Applicability
This document is applicable to all role players, activities and processes involved in the production of
elevation data capture and DEM generation within the CD: NGI.
A 6.5 Assumptions
It is assumed that the provisions of this document may be applied by users outside CD: NGI.
A 6.6 Normative References, other standards and related documents
The document Recommended Practice for the Structure and Drafting of Standards and
Related documents is used to guide the format and structure of this document.
The National Standard for the Exchange of Digital Geo-Referenced Information
Geographic information – Spatial referencing by coordinates - ISO/FDIS 19111 : 2002 (E)
(final draft)
Spatial Data Infrastructure Act 54 of 2003
SCOP 3.3 Introductory Class of 1998
The South African National Digital Elevation Model of 1997
A 6.7 Maintenance Authority
Maintenance
Maintenance of the Standard for Digital Elevation Model Data is the responsibility of the Division:
Quality Assurance of CD: NGI. Changes to this Standard will be instructed by the Chief Director as
improvements or amendments become necessary, or as required. Any request for amendments to
this Standard may be submitted by any institution, body or individual to the Chief Director for
consideration. All such requests and any other comments on the Standard must be addressed to:
Chief Director: National Geo–spatial Information
Private Bag X10
Mowbray
7705
and be referenced as: Amendment - Standard for Digital Elevation Model Data
The Division: Quality Assurance shall maintain the provisions and structure of this document
through amendment and revision activities.
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A 6.8 Roles and Responsibilities
The roles and responsibilities of the main role players as it pertains to the drafting and maintenance
of this document are stated below.
A 6.8.1
Division: Quality Assurance
To maintain the provisions of this document.
Keeping track of all amendments to this document
Providing assistance and guidance to the Standards Development Committees and CD:
NGI Management in interpreting the provisions of this document
Provisionally approving a proposed revision of this standard with respect to its structure and
format.
Ensuring that the use and purpose of this document is communicated effectively.
A 6.8.2
Standards Development Committee – Standard for Digital Elevation Model Data
To draft revised specifications to the provisions of this document
To make recommendations on the update and amendment of the provisions of this
document
Refer to the relevant section in the Terms of Reference of the Standards Development
Committee for more details in this regard.
A 6.8.3
Geomatics Manager: Quality Assurance
Evaluate proposed amendments and revisions of this Standard.
Provisionally approving proposed amendments to this Standard.
Refer to the relevant section in the Terms of Reference pertaining to the Geomatics
Manager: Quality Assurance for more details in this regard.
A 6.8.4
CD: NGI Senior Management
To approve, amendment and revise this Standard.
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SECTION B
Normative Elements
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B1
Digital Elevation Data
B 1.1 General
(a) DEM data is represented in the form of a rectangular grid of planimetric coordinate
pairs (y, x) and elevation (H) for each grid point.
(b) DEM generated by means of the specifications stated below, shall be referenced to a
compound coordinate reference system (CCRS).
(c) The rectangular grid shall be referred to as DEM and shall be supplied in an ORT
(orthogonal profiles) file format, an ASCII file containing coordinate pairs (y,x) and
elevation (H).
B 1.2 Elevation Accuracy
B 1.2.1 25m DEM (supplied as ORT file format)
(a)
(b)
(c)
The 25m DEM shall be derived from analogue aerial photography at an approximate
scale equal to or greater than 1:32 000, scanned at 25 microns (equivalent to 1016
DPI), or digital aerial photography at a GSD equal to or less than 0.5m (photography
that is usually used for the 1:10 000 orthophoto rectification and map production.
The standard error for heights shall be 1.2m (σ) and 2.5m (2σ) in flatter areas, and 2m
(σ) and 4m (2σ) in steeper areas of the country where DEM is available.
Any other DEM compiled from source other than stated in B 1.2.1 (a) shall have the
following accuracy:
σ
2σ
Photogrammetrically compiled 5m contours
1.4m
2.9m
Vectorised 5m contours transformed to WGS84
4.0m
8.1m
Vectorised 20m contours transformed to WGS84
5.2m
10.5m
Source
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B2
Digital Elevation Model
B 2.1 DEM Structure
(a) The planimetric coordinates (y,x) contained in the data set shall be referenced to the
Hartebeesthoek94 Datum for South Africa. The height (H) shall be referenced to MSL.
(b) The coordinates of the data sets stated in B 2.1 (a), shall be based upon the South
African Coordinate System, Gauss Conform Projection (Hartebeesthoek94-WGS84).
(c) The rectangular grid representing part of the earth surface shall be structured as y, x
coordinates and H elevations as profile lines.
(d) Each profile line shall start in the South-West corner of the area of profiling, moving
Northwards and each successive profile shall be added in an easterly direction.
(e) The rectangular grid shall be formatted according to a “saw-tooth” methodology i.e. the
profiles in the grid follow a “saw-tooth” pattern as indicated in Figure B 2.1.
Figure B 2.1 - DEM Structure according to the “saw-tooth” pattern
(Clarke et al 1990, p.131)
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B 2.2 Output Format
B 2.2.1. 25m DEM ORT file format
(a) The data of the ORT file shall be structured according to the example given in Figure B
2.2 (a) to adhere to the relevant requirements stated in B 2.1.
(b) The data shall be structured within the ORT file in such a manner that the planimetric
coordinates and MSL height values read from left to right, and are divided into three
discreet columns where the first column indicates the y planimetric coordinate values,
the second column indicates the x planimetric coordinate values, and the third column
indicates the H value of the point.
(c) The columns referred to in B 2.2.1 (b) and listed in Figure B 2.2 (a) shall be space
delimited so that columns of the various DEM element values may be distinguished.
(y) (x)
(H)
97200 3270925 1113.27
97200 3270900 1113.42
97200 3270875 1113.54
97200 3270850 1113.69
97175 3270925 1113.90
97175 3270900 1114.03
97175 3270875 1114.18
97175 3270850 1114.33
97150 3270925 1114.73
97150 3270900 1114.80
97150 3270875 1114.95
97150 3270850 1115.11
(Start of Profile 1)
(End of Profile 1)
(Start of Profile 2)
(End of profile 2)
(Start of Profile 3)
(End of Profile 3)
NB. Notes contained in parentheses are for information purposes only
Figure B 2.2 (a) - Example of Profile lines stored in an ASCII file with extension *.ORT
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B 2.2.2. Naming convention for DEM ORT files
(a) The DEM file naming convention shall carry general metadata elements, which are
delimited by a “_” in the file name, an example is as follows:
LO25_050M_2925BA01.ORT
LO25 – the central meridian coordinate system ( Gauss Conform projection)
050M – the absolute accuracy of the DEM is 2.5m – 5m
2925BA01 – the sheet reference index, in this case the 1:10 000 reference index.
ORT – ASCII file name extension, orthogonal profiles.
(b)
Typical metadata accuracy element shall include the following.
Accuracy Description
0M TO 2.5M
2.5M TO
5M
5M TO 10M
10M TO 20M
20M TO 30M
30M TO 40M
40+M
B3
Accuracy Code
025M
050M
100M
200M
300M
400M
500M
Elevation Data Generation
B 3.1 Mass Elevation Data Capture Requirements
B 3.1.1 System and correlation requirements
(a) Only full 3-dimensional digital photogrammetric workstations (DPW) with full georeferencing capabilities shall be used for mass elevation data capture.
(b) The mass elevation point data shall be automatically interpolated using image
correlation techniques (pixel to pixel matching) and all elevation points interpolated
above ground shall be edited to ground.
B 3.1.2 Density and distribution of mass elevation point capture
The distribution of sampled mass elevation data points shall not have a spacing of more
than 50m in both North/South and West/East direction from any sampled point.
B 3.1.3 Breaklines and Formlines
(a) Formlines and breaklines shall be manually captured as line geometry with elevation
embedded.
(b) Breaklines shall be captured along all river, road and railway centre lines, dam walls
and dam water levels.
(c) Formlines that describe the line structure of the terrain, e.g. lines of ridges and valleys
where terrain does not break, shall be captured along all ridges, cuts and fills (diggings
and embankments) and steep shadow areas where mass elevation placement is
difficult.
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B 3.1.4 Spotheights
(a) Spotheights shall be placed in major saddles, on major hills and in areas where the
contours do not give a clear indication of the surrounding terrain (flat areas or on the
edge of map sheet boundaries).
(b) In areas where the contours do not give a clear indication of the surrounding terrain (flat
areas or on the edge of map sheet boundaries), the spotheight point may be placed on
identifiable terrain objects such as spillway of dams and intersections of roads and
fences.
B 3.2 Contour Generation
B 3.2.1 Contour Generation Requirements
(a) Contours shall be generated by automatic interpolation that infers breaklines, formlines
and ridgelines in its calculation. The data set shall contain sufficient mass elevation
point data for automatic contour generation together with formlines, breaklines,
ridgelines and spotheights.
(b) All mass elevation point data that correlates above ground shall be edited to ground
level before contour generation.
(c) All formlines, breaklines, ridgelines and spotheights shall be manually captured.
(d) Contour-generated outputs shall be line geometry with elevation data embedded within.
(e) Unless otherwise stated the contour interval shall be 5m for both standard and
depression contours.
(f) All contours shall turn accurately on river formlines, shall be continuous line segments,
shall have elevation embedded and shall be approximated along dam walls, where they
are of the same height, or lower, than the dam’s full supply level.
B 3.1.2
Contour Accuracy
(a) The contour accuracy shall be directly dependant on the accuracy of the aerial
triangulation and image correlation.
(b) Aerial Triangulation data accuracies shall be in accordance with the Image Absolute
Orientation Standard.
(c) 95% of all contouring shall be within half of the contour interval of the absolute ground
level.
(d) Absolute ground level shall be determined by the elevation of trigonometrical beacons,
photo ground control points and benchmarks.
B 3.3 DEM Generation Requirements
(a) Only linear prediction methods of interpolation shall be used for the computation of
DEM data, any method averaging neighbouring points may not be used. The prediction
method shall include both Bell Curve and Straight Line base functions. The Bell Curve
function shall be used in most cases, the Straight Line base functions may be used
where there are sudden changes in terrain slope throughout the subject area.
(b) The interpolation algorithm shall infer breaklines, formlines and spotheights in its
calculation.
(c) Mass Elevation Data, for DEM generation, shall include the sampled/correlated terrain
ground points, spotheights, breaklines, formlines and contours.
(d) DEM-generated output shall be a 25m regular rectangular grid.
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SECTION C
Supplementary Informative Elements
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C1
References
Clarke, D.G., Cooper, A.K., Liebenberg, E. C. & van Rooyen, M.H., 1990. A national standard for
nd
the exchange of digital geo-referenced information. 2 ed. South Africa: CSIR.
Clarke, D.G., 1997. The South African National Digital Elevation Model. [Unpublished Paper] South
Africa: Chief Directorate Surveys Mapping
INPHO GmbH, 1998. SCOP 3.3 Introductory Class. [Unpublished Paper] Stuttgart: INPHO GmbH
International Standard Organisation, 2002. Geographic information – Spatial referencing by
coordinates. Geneva: ISO
South African Government, 2003. Spatial Data Infrastructure Act 54 of 2003. [Act] South Africa:
Juta & Company Ltd.
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