Geomatics guidance note 20 International Association of Oil & Gas Producers

Coordinate reference systems for the Arctic

Revision history Report No.

Version

Date

Amendments

373-20

1.0

July 2013

First release

1. Introduction This guidance note discusses issues associated with mapping at high latitudes and provides recommendations for coordinate reference systems (CRSs) for petroleum exploration and production in the Arctic. It is aimed at geomatics specialists, other geoscientists and engineers.

2. Map Projections — Considerations for the Arctic 2.1 General considerations A fundamental issue regarding the choice of coordinate reference system in the Arctic concerns the map projection method. As outlined in OGP Geomatics guidance note 1 (2007), the act of projection introduces distortion. The distortion characteristics are exacerbated at high latitudes. A detailed discussion on map projection distortion and consequent method selection is beyond the scope of the guidance note; there are numerous text books on the subject, such as Datums and Map Projections (Iliffe & Lott, 2012). Key issues that should be considered in selection of a map projection include: geographic extent, particularly latitude range; software capabilities; and later use of the data. These issues are relevant globally but map projection characteristics make them especially important in the Arctic.

Geographic extent

Geographic extent is measured in terms of latitude and longitude of the area of interest. One degree of latitude is approximately 110 km; this value changes only slightly with absolute latitude. At the equator one degree of longitude is also approximately 110 km but because of the convergence of meridians towards the pole it decreases with latitude to 55 km at 60°N, 30 km at 74°N, 20 km at approximately 80°N and zero at the pole. The Transverse Mercator (TM) map projection method, which is the basis for the Universal Transverse Mercator (UTM) grid system, is best suited to areas of significant latitude extent and limited (up to 8°) longitude extent. Conversely the Lambert Conic Conformal (LCC) map projection method is best suited to areas of limited latitude extent and significant longitude extent. Using the same constraint on maximum scale error at high latitudes LCC zones can cover larger areas than would be the case using TM. Consequently, northwards of 60°N an LCC map projection would normally be chosen in preference to a TM map projection. The absolute value of latitude

Near the geographic North Pole the mathematics used in popular map projection methods (TM, LCC) break down. When the area of interest includes areas north of 85°N it is normal to consider a polar azimuthal map projection method (for example one of the Polar Stereographic map projection methods).

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The support for the map projection method offered by software applications

When an area of interest is oblique to parallels and meridians, for example offshore southeast Greenland or offshore Norway, in principle a map projection using the Oblique Mercator method could cover the whole area in a single zone. However this map projection method is not always supported in geoscience and engineering applications and therefore may not be an appropriate choice. For the Lambert Conic Conformal and Polar Stereographic map projection methods, there are variations in the set of parameters used to define a zone; it is therefore essential to choose the variant supported by the application. The capabilities and geodetic integrity of geoscience software may be evaluated following the procedures described in OGP report №430 (2011a). Later uses of the data

There is a tendency for CRSs which are satisfactory for the initial purpose to continue to be used for follow-up projects, even if they are unsuitable. This implies that use beyond the immediate project should be considered whenever possible and a coordinate reference system suitable both for the current project and anticipated further projects should be chosen. As an example, a seismic binning grid inherits the properties of the projected CRS upon which it is based, including and in particular, the map projection distortion. It is not possible to reproject the bin grid into a different CRS and maintain the shape and regular spacing of the bins because the distortion characteristics of the two CRSs differ. Figure 1 shows the reprojection of the perimeter of a bin grid. The dashed blue rectangle represents the size and shape of the original definition, the green polygon is that shape converted to the new CRS. Issues associated with this problem are discussed in the P6/11 format user guide (OGP, forthcoming). To avoid reprojection problems it is therefore best that the seismic data is acquired and binned in the CRS appropriate for development work in the area. The choice of map projection during exploration should, if possible, also satisfy the needs of development. Facilities engineering requirements are that the map grid preserves correct shape. This is met through a ‘conformal’ map projection method. Engineering also requires that scale be true. This limits the area over which methods such as TM (including UTM) and LCC can be used to approximately 6° longitude for TM and 5° latitude for LCC.

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Figure 1: Reproduction of a seismic bin grid

A

D

B

C

2.2 Considerations specific to the Lambert Conic Conformal method This is the map projection method of choice for Arctic studies where longitude extent (in degrees) exceeds latitude extent and the area of interest excludes extremely high latitudes (north of 87°N). If the latitude range can be restricted to no more than 5°, the scale distortion will be similar in magnitude to that of the UTM grid system. Two standard parallels (2SP) that divide the latitude extent of the area 1/6 – 2/3 – 1/6 should be considered. For example, a project at 70°N might use a Lambert Conic Conformal (2SP) map projection with standard parallels at 68°30'N and 71°30'N; this would be satisfactory between 67°30'N and 72°30'N¹. A further advantage of the LCC map projection method is that at high latitudes its formulae are mathematically more stable than those of the TM map projection method. As described in OGP Geomatics guidance note 7, part 2 (2013a) there are several variations of the LCC method. The definition in the previous paragraph is termed ‘Lambert Conic Conformal (2SP)’ in section 1.3.1.1 of guidance note 7-2. Although it is possible to define an equivalent map projection using a single standard parallel (‘Lambert Conic Conformal (1SP)’ in section 1.3.1.2 of note 7-2), would-be users should be aware that some software applications do not support this method. Nor do many low-cost GPS receivers. Consequently the Lambert Conic Conformal (2SP) method is to be preferred. For these reasons the systems outlined in the Appendix incorporate Lambert Conic Conformal (2SP) projections in their definitions. ¹N.B. for an ellipsoidal projection the minimum scale on a Lambert Conic Conformal projection is a few kilometres poleward of the latitude midway between the two standard parallels. In this example the minimum scale is approximately 0.9996574 at approximately 70°01'05.07"N.

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2.3 Considerations specific to the Transverse Mercator method The map projection method most commonly encountered in the petroleum industry is the Transverse Mercator, also called Gauss-Kruger. It is used as the basis for the UTM (Universal Transverse Mercator) zoned projection series. The equations used in the map projection method fail at very high latitudes. Furthermore the map projection mathematics are optimised for areas of largely northsouth extent. Conversely, the distortions inherent in a TM map projection become significant with east-west extent. To maintain scale distortion at under 0.5 metres per kilometre (scale factor between 0.9995 and 1.0005) a TM map projection should be used within 270 kilometres either side of its longitude of origin (central meridian). This limits the zone width to ±4.5° longitude at 60°N, ±7° longitude at 70°N and ±14° longitude at 80°N. However, once the longitude extent exceeds ±4° longitude it is critical that coordinate conversion, scale factor and grid convergence algorithms are optimized for wide area usage. Over 4° longitude from the central meridian, commonly encountered (‘standard’) TM map projection equations will only awude/longitude to grid then grid back to latitude/ longitude. Repeated use will cause data to ‘move’ as the approximations become greater. To mitigate against this, the JHS formulae (see section 1.3.5 of guidance note 7-2, OGP, 2013a) should be used. Although the use of these wide area formulae will minimise numeric accuracy problems, significant distortions in distance and area can only be avoided by limiting map projection usage as described in the previous paragraph. Note that the algorithms built into most E&P industry software applications use less robust mathematics, typically similar to the USGS formula described in section 1.3.5 of guidance note 7-2 (OGP, 2013a). If using ‘standard’ formula, the practical use of the TM map projection for petroleum production and construction engineering purposes should be limited to no more than ±4 degrees of longitude from the longitude of origin (central meridian). At high latitudes the convergence of meridians restricts the linear distance (d) over which the map projection should be used. As a rough guide a TM map projection using‘standard’ formula should be used only within d = +/−(450 cos ϕ) kilometres either side of its longitude of origin (central meridian), where ϕ is the latitude. This limits the full zone width to 300 km at 70°N, 230 km at 75°N, and 150 km at 80°N.

Data to test whether ‘standard’ or wide area TM formulae are built into application software is given in the test dataset accompanying OGP report №430 (2011a).

2.4 Considerations specific to the Polar Stereographic method The Polar Stereographic map projection method may be defined through one of several variations (refer to section 1.3.7.2, guidance note 7-2, OGP, 2013a) and few software applications can cope with all of these variations. It is necessary to check which particular variation is included in the computer application(s) to be used. The choice of longitude of origin for a Polar Stereographic map projection dictates the map orientation of features such as coastline. Areas adjacent to the longitude of origin appear in the familiar “north up” orientation. Areas with longitudes 180° from the longitude of origin will appear “upside down”. The choice of longitude of origin therefore may be used to constrain the areas that appear “the right way up”. See figure 2.

Figure 2: Polar Stereographic orientation

Alaska-centric EPSG CRS 5936

Norway-centric EPSG CRS 5939

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2.5 Grid convergence Grid convergence is the angle between the directions to true north and to grid north in a map plane. Its exact value varies from point to point and differs between map projection methods and zone definitions. However for all map projections the value of grid convergence is of the same order of magnitude as the change in longitude from the projection origin. At high latitudes large grid convergence cannot be avoided. It therefore must be corrected for. Grid convergence is critically important for the correct positioning of deviated wellbores, and may have relevance in long offset seismic acquisition. There are two opposing conventions for the sign of grid convergence. This is discussed further in OGP Geomatics guidance note 21 (2013b). The rate of change of grid convergence increases with latitude. Despite this, the effect caused by the variation of grid convergence over short distances is generally not significant. In theory the changing value of grid convergence should be applied along a well bore. But an assumption that a single value of grid convergence can be applied uniformly along the full offset may be adequate even at high latitudes. For example, for a long reach well of 25 kilometres at 70°N, application of the value of grid convergence at the well head to the whole well path will give rise to an error of no more than approximately 3 metres.

3. Coordinate systems For circumpolar projections the terms ‘easting’ and ‘northing’ are not meaningful geographically. At the North Pole both axes are orientated southwards. Instead the familiar letters ‘X’ and ‘Y’ are recommended. The X-axis should be defined to be 90° clockwise from the Y-axis, when viewed from above.

4. Geodetic datum Coordinate Reference Systems based on modern national geodetic datums such as NAD83, GR96 and ETRS89 should normally be used. For regional studies, which may transcend national boundaries, the use of WGS 84 is acceptable. See OGP Geomatics guidance notes 4 (2002) and 19 (2011d) for further advice. Certain microwave satellite imagery products used for polar ice studies utilise the Hughes ellipsoid. This has no geodetic basis. Consequently positions derived from the imagery are 4

suitable only for very small scale (large area) studies, such as monitoring relative changes in ice coverage. For the derivation of gravity-related heights from 3-dimensional ellipsoidal coordinates, the national height correction or geoid model should be used. If no national model is available, application of the global EGM2008 geoid model is recommended.

5. Legacy data The recommendations in section 6 post-date some Arctic exploration. Consequently it may be necessary to convert or transform the coordinates of legacy data before it can be merged with modern data.

6. General recommendations 6.1) In the immediate vicinity of the North Pole (north of latitude 87°N) and for whole-Arctic studies, a map projection using the Polar Stereographic method should be used. 6.2) South of 87°N but north of the Arctic Circle (approximately 66°30'N), for exploration and development work a map projection using the least scale distortion such one utilizing the Lambert Conic Conformal (LCC) method is recommended. 6.3) For exploration and development work, geodetic coordinate reference systems which are a national realization of the ITRS should be used, for example NAD83(NSRS) in Alaska, NAD83(CSRS) in Canada, GR96 in Greenland, ETRS89 in Norway. 6.4) To avoid reprojection problems 3D seismic data should be acquired and binned in the coordinate reference system appropriate for development work in the area. 6.5) For work in the Arctic the projected coordinate reference systems in the Appendix are recommended. These incorporate the above recommendations. The definitions are available in version 8.2 and later of OGP’s EPSG Geodetic Parameter Dataset (EPSG).

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References EPSG Geodetic Parameter Dataset, http://info.ogp.org. uk/geomatics and www.epsg-registry.org Iliffe, J and Lott, R (2012 reprint) Datums and map projections, Whittles Publishing. OGP (2002) Survey & positioning guidance note 4: Use of the International Terrestrial Reference Frame (ITRF) as the reference geodetic system for surveying and real-time positioning, OGP report №373-04, available from http://www.ogp.org.uk/pubs/373-04.pdf OGP (2007) Survey & positioning guidance note 1: Geodetic awareness, OGP report №373-01, available from http://www.ogp.org.uk/pubs/373-01.pdf OGP (2011a) Geospatial integrity of geoscience software part 1: GIGS guidelines, OGP report №430-1, available from http://www.ogp.org.uk/pubs/430-1.pdf OGP (2011b) Geospatial integrity of geoscience software part 2: GIGS software review, OGP report №430-2, available from http://www.ogp.org.uk/pubs/430-2.pdf OGP (2011c) Geospatial integrity of geoscience software part 3: User guide for the GIGS Test Dataset, OGP report №430-3, available from http://www.ogp.org.uk/pubs/430-3.pdf OGP (2011d) Geomatics Guidance note 19: Guidelines for GNSS positioning in the oil & gas industry, OGP report №373-19, available from http://www.ogp.org.uk/pubs/373-19.pdf OGP (2013a) Geomatics Guidance note 7 part 2: Coordinate conversions and transformations including formulas, OGP report №373-7-2, available from http://www.ogp.org.uk/pubs/373-07-2.pdf OGP (2013b) Geomatics guidance note 21: Grid Convergence, OGP report №373-21, available from http:// www.ogp.org.uk/pubs/373-21.pdf OGP (forthcoming) OGP P6/11 Seismic bin grid data exchange format – user guide, OGP report №483-6u.

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Appendix: EPSG Arctic map projections and CRSs The coordinate reference systems recommended for use in the Arctic are based on three series of map projections outlined below. The definitions for the projections and CRSs of all three series are available in OGP’s EPSG Geodetic Parameter Dataset (see References).

Figure 3: EPSG Arctic Regional LCC zones

A.1 EPSG Arctic Polar Stereographic zones EPSG CRS codes 5936 to 5940. These five conformal map projections and CRSs each cover the whole Arctic. Each zone is identical in definition to the Universal Polar Stereographic projection but rotated to offer a ‘north up’ map view of the area as seen from a continental perspective. The five views represented are Alaska-, Canada-, Greenland-, Norway- and Russia-centric. The Alaskan and Norwegian views are shown in figure 2.

A.2 EPSG Arctic Regional Lambert Conic Conformal zones EPSG CRS codes 5921 to 5935. These zoned conformal map projections and CRSs are intended for regional studies including small scale mapping of very large areas. They are unsuitable for exploration and development field work. Three overlapping bands (A through C) each with latitude extent of 12° circle the Arctic and sub-Arctic south of 87°N (see figure 3). Within each band there are five overlapping zones, each covering a nominal 90° in longitude but which may be extended eastwards or westwards as required.

Bands A and C

Band B

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A.3 EPSG Arctic LCC zones

Figure 4: EPSG Arctic LCC zones

EPSG CRS codes 6050 to 6093, 6098 to 6125 and 6351 to 6354. These map projections and CRSs are intended for exploration and development work including engineering. Six overlapping bands (1 through 6) cover the Arctic south of 87°N, with a further two bands (7 and 8) covering sub-Arctic areas. Each band has latitude extent of 5° and overlaps its neighbouring band by 1°40' or one third of the band width. Within each band is a series of zones extending approximately 800 km in an east-west direction, equivalent to approximately 15° in longitude in band 7 and increasing to approximately 60° in longitude in band 1. Zones may be extended eastwards or westwards as required. The band configurations are shown in figure 4. These Arctic LCC zones are used south of 87°N. For detailed spatial analysis and mapping in proximity to the North Pole (north of 87°N), a Polar Stereographic map projection will be required. This has not been defined at the present time. Bands 1, 3, 5 and 7

Bands 2, 4, 6 and 8 [outermost]

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