Grid Shift Files

Grid shift files are applied for various datum transformations.  They are commonly used to transform coordinates from an older reference frame to a newer reference frame, which is usually more accurate.  The transformation cannot improve the precision of the coordinates, but by  moving to a more accurate reference frame, it enables the future integration of higher resolution and more precise geospatial information.  In Canada, grid shift files serve mainly to transform coordinates between different realizations of the North American Datum (NAD).  Currently, the CGRSC agencies make available tens of files defining hundreds of grid areas throughout Canada.

A standard format was initially designed to support the transformation of horizontal (2D) coordinates between NAD27 and NAD83(original).  The format includes header records that describe the coverage area and spacing interval that apply to all grids embedded in a file.  In Canada, the accompanying NTv2 application is used to search these grids and interpolate the coordinate shifts of four grid points nearest a user-specified location.

Grid shift files are usually provided in a binary format with the extension GSB. There is also an ASCII representation of this file, which has the extension GSA. A single GSB file can contain several grids containing coordinate shift values at node points. Higher resolution grids for targeted areas can also be embedded and related to the lower density ones.  Although originally developed by NRCan’s Canadian Geodetic Survey for national transformation between NAD27 and NAD83(Original), some provincial agencies have developed their own grid shift files to support regional transformations. Table 1 lists available national and provincial grid shift files and the extent of their coverage.  Users should always be mindful when applying a grid-based transformation because of the uncertainty they may introduce in their working area.

Table 1. Summary of grid shift files available in Canada

 

 

Grid

Sub Grids

 

F/P

File

 

Transformation

Latitude

Longitude

Interval

#

Interval

CA

NTV2_0

NAD27

NAD83 (ORIG)

4000’-6000’

4400’-8800’

5’x5’

8

30”x30”

   

NAD27

NAD83 (ORIG)

4700’-6000’

8800’-14200’

5’x5’

91

30”x30”

   

NAD27

NAD83
(ORIG)

6000’-7500’

4400’-14200’

5’x10’

11

30”x30”

   

NAD27

NAD83
(ORIG)

7500’-8400’

4400’-14200’

5’x20’

0

 
                 

AB

ABCSRSv4

NAD83

CSRS 2002

4855’-6005’

10955’-12005’

5’x5’

56

30”x30”

BC

BC_27_98

NAD27

CSRS 1997

4700’-6200’

11200’-14000’

5’x5’

23

15”x15”

 

BC_CSRS

NMIP93

CSRS 1997

4700’-6200’

11200’-14000’

5’x5’

23

15”x15”

 

BC_27_05

NAD27

CSRS 2002

4835’-6025’

11420’-13805’

5’x5’

18

10”x10”

 

BC_93_05

NAD83

CSRS 2002

4835’-6025’

11420’-13805’

5’x5’

18

10”x10”

 

PRF2005a

NAD83

CSRS 2002

4835’-6025’

11420’-13805’

5’x5’

18

10”x10”

 

CRD27_00

NAD27

CSRS 1997

4815’-4900’

12300’-12430’

1’x1’

1

15”x15”

 

CRD93_00

NMIP93

CSRS 1997

4815’-4900’

12300’-12430’

1’x1’

1

15”x15”

 

CRD98_00

CSRS (v2) 1997

CSRS (v3) 1997

4815’-4900’

12300’-12430’

1’x1’

1

15”x15”

 

NVI27_05

NAD27

CSRS 1997

4830’-5100’

12330’-12910’

5’x5’

7

10”x10”

 

NVI93_05

NMIP93

CSRS 1997

4830’-5100’

12330’-12910’

5’x5’

7

10”x10”

 

NVI98_05

CSRS (v2) 1997

CSRS (v3) 1997

4830’-5100’

12330’-12910’

5’x5’

7

10”x10”

MB

               

NB

NB2783v2

NAD27

CSRS 1997

4400’-4830’

6300’-6930’

30”x30”

0

 
 

NB7783v2

ATS77

CSRS 1997

4400’-4830’

6300’-6930’

30”x30”

0

 

NL

               

NS

GS7783

ATS77

NAD83 (ORIG)

4315’-4830’

5905’-6910’

5’x5’

0

 
 

NS778301

ATS77

CSRS 1997

4230’-4800’

5830’-6800’

2’x2’

0

 
 

NS778302

ATS77

CSRS 2010

4300’-4730’

5900’-6700’

30”x30”

0

 

ON

ON27CSV1

NAD27

CSRS 1997

4000’-5800’

7300’-9800’

5’x5’

8

30”x30”

 

MAY76V20

MAY76

NAD83 (ORIG)

4000’-6000’

4400’-8800’

5’x5’

8

30”x30”

 

ON76CSV1

MAY76

CSRS 1997

4000’-5800’

7300’-9800’

5’x5’

8

30”x30”

 

ON83CSV1

NAD83

CSRS 1997

4000’-5800’

7300’-9800’

5’x5’

8

30”x30”

 

TOR27CSV1

NAD27

CSRS 1997

4334’-4352’

7907’-7939’

5”x5”

0

 

PEI

PE7783V2

ATS77

NAD83
(ORIG)

4545’-4705’

6135’-6440’

30”x30”

0

 

QC

NA27SCRS

NAD27

CSRS 1997

4455’-6300’

5600’-8000’

5’x5’

0

 
 

NA83SCRS

NAD83

CSRS 1997

4455’-6300’

5600’-8000’

5’x5’

13

30”x30”

 

CQ77NA83

CQ77

NAD83
(ORIG)

4455’-6255’

5600’-7955’

5’x5’

0

 
 

CQ77SCRS

CQ77

CSRS 1997

4455’-6300’

5600’-8000’

5’x5’

0

 

SK

SK27-83

NAD27

NAD83
(ORIG)

4800’-6100’

9900’-11200’

5’x5’

25

30”x30”

 

SK27-98

NAD27

CSRS 1997

4800’-6100’

10100’-11100’

5’x5’

18

30”x30”

 

SK83-98

NAD83

CSRS 1997

4800’-6100’

10100’-11100’

5’x5’

16

30”x30”

1 F/P: Federal/Provincial source
2 CSRS: NAD83(CSRS)

As the quality of coordinate transformations between reference frames will vary depending on the precision and distribution of the control stations used to estimate grid shift values, it is important to remember precision limitations and possible variability within a specified area of coverage.  Table 2 gives estimates of the average magnitude of the grid shift values and their uncertainty over different coverage areas.  Nominally, a municipal grid would cover an area of 100  km2 (e.g.,  Greater Toronto Area), a regional grid can cover an area of 10,000 km2 (e.g., Victoria Capital Regional District), and a provincial/territorial grid can cover an area of over 1,000,000 km2. The precision estimates reported in the table are average values from various analysis conducted by CGRSC agencies in targeted regions.  Table 2 gives a few links to these analyses.

Table 2: Average spacing and precision of three grid shift files for different areas in Canada

    Municipal Regional Provincial (South) Provincial (North) Territorial
Grid Spacing   5”x5” 30”x30” 5’x5’ 5’x5’ 5’x10’-20’
Cell Size   0.02 km2 1 km2 50 km2 40 km2 50-100 km2
             
Transformation Shift (m) Average Precision (cm)
NAD27-NAD83 0-100 10 15 15 50 100
NAD27-CSRS 0-100 3 10 10 50 100
NAD83-CSRS 0-2 3 5 5 50  

http://www2.unb.ca/gge/Personnel/Santos/Santos_SNB_Report_Grid_Shift-NAD27-NAD83.pdf

http://geonova.novascotia.ca/sites/default/files/resource-library/NSCRS%20Technical%20Support%200009%20Grid%20Shift%20Files%203Feb2016.pdf

http://www1.toronto.ca/City%20Of%20Toronto/Engineering%20and%20Construction%20Services/Standards%20and%20Specifications/Files/pdf/NAD%2083/nad_83_csrs.pdf

Transformation between NAD27 and NAD83(Original)

This grid-based transformation is performed using national or provincial grid shifts files with the National Transformation version 2 (NTv2) application provided by NRCan. NTv2 is a national standard for transforming horizontal coordinates (geographic or UTM/MTM/Stereographic) between NAD27 and NAD83(Original) reference frames.

Figure 1 shows a national grid of coordinate differences between NAD27 and NAD83(Original). The figure illustrates that the systematic shifts between the two reference frames (datums), although consistent over a small area, can be different by more than 100 metres across the country. This systematic shift originates from the fact that NAD27 and NAD83(Original) are based on different ellipsoids (Clarke 1866 and GRS80) with their origins located more than 200 metres apart.
Horizontal shifts between NAD27 and NAD83(Original) coordinates

Figure 1. Horizontal shifts between NAD27 and NAD83(Original) coordinates.

Transformation between NAD83(Original) and NAD83(CSRS)

For NAD83(Original) and NAD83(CSRS), it is important to understand that the former is a 2D reference frame (horizontal datum) while the latter is a 3D geometric reference frame.  The transformation of the horizontal coordinates between NAD83(Original) and NAD83(CSRS) is only possible where provincial agencies developed regional grid shift files (see Table 1 in section ‘Grid Shift Files’).  There is no national grid shift files because the control stations are too sparse to represent the irregular patterns of the distortions.  Figure 2 shows vectors of horizontal coordinate differences between NAD83(Original) and NAD83(CSRS) at the Canadian Base Network (CBN) stations. One can notice significant changes in vector orientation and magnitude across different regions of the country.  On average, the horizontal differences are about 30 cm, but they can exceed 1 m in some provinces.

Horizontal differences between coordinates of the CBN points in NAD83(Original) and NAD83(CSRS) reference frames

Figure 2. Horizontal coordinate differences between NAD83(Original) and NAD83(CSRS) reference frames at the CBN stations.