SINGLE MASTER GRAVITY AND AEROMAGNETIC DATA FOR ONTARIO
Ontario Geological Survey
Ministry of Northern Development and Mines
Willet Green Miller Centre
933 Ramsey Lake Road
Sudbury, Ontario, P3E 6B5, Canada
ERLIS Dataset CD-ROM # 1035 (ASCII Format) and # 1036 (Geosoft Format)
Issued July 12, 1999
TABLE OF CONTENTS
1. BOUGUER GRAVITY AND VERTICAL GRAVITY GRADIENT OF ONTARIO.................... 1
1.1 INTRODUCTION .............................................................................................................. 1
1.2 GRAVITY DATA PROCESSING ..................................................................................... 1
1.2.1 Editing.................................................................................................................... 1
1.2.2 Gridding................................................................................................................. 2
a) The Bouguer Gravity Grid
b) The Vertical Gravity Gradient Grid
1.2.3 Grid Projection....................................................................................................... 3
1.3 DATA SPECIFICATIONS................................................................................................. 4
2. TOTAL MAGNETIC FIELD AND VERTICAL MAGNETIC GRADIENT OF ONTARIO ....... 4
2.1 INTRODUCTION .............................................................................................................. 4
2.2 MAGNETIC DATA PROCESSING.................................................................................. 5
2.2.1 Editing.................................................................................................................... 5
2.2.2 Levelling ................................................................................................................ 6
2.2.3 Gridding................................................................................................................. 6
a) The Total Magnetic Field Grid
b) The Vertical Magnetic Gradient Grid
2.2.4 Grid projection....................................................................................................... 7
3. ADDITIONAL DATA PROCESSING ........................................................................................... 7
3.1 INTRODUCTION .............................................................................................................. 7
3.2 GRAVITY DATA .............................................................................................................. 7
3.3 DIGITIZED MAGNETIC DATA ...................................................................................... 8
3.4 DIGITALLY RECORDED MAGNETIC DATA .............................................................. 8
REFERENCES ............................................................................................................................................ 9
CREDITS.................................................................................................................................................... 11
SOURCES OF INFORMATION................................................................................................................ 11
APPENDIX A- CONTENTS OF PROFILE AND GRID DATA FILES .................................................. 12
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1. BOUGUER GRAVITY AND VERTICAL GRAVITY GRADIENT OF ONTARIO
1.1 INTRODUCTION
The systematic regional gravity mapping of Ontario was initiated in 1947, by the Dominion
Observatory of Canada (Innes, 1960). By 1964, uniform survey coverage of most of the province had
been achieved, at a relatively coarse station spacing of 10 to 15 km, and the resulting Bouguer anomaly
maps published at a scale of 1:500,000. Later surveys by the Earth Physics Branch of Energy, Mines and
Resources, Canada, increased the density of coverage in certain areas of the province.
The metavolcanic-metasedimentary belts of Ontario are perceived to be regions of high economic
mineral potential. Prospecting and geological mapping in these regions are hampered by inaccessibility,
difficult terrain, and extensive areas of thick glacial overburden. The importance of gravity data as a
reconnaissance tool in such areas led the Ontario Division of Mines, in 1970, to initiate systematic,
detailed gravity surveys of these belts. The results of the surveys have been used to outline areas
warranting detailed follow-up exploration (Gupta and Sutcliffe, 1990), and have proved an effective aid to
geological mapping (Gupta and Ramani, 1982).
These systematic detailed gravity surveys have helped to outline the deeper geological and
geophysical characteristics of the Precambrian Shield of Ontario (Gupta and Ramani, 1980; Gupta and
Grant, 1985). Gravity interpretation and modelling have constrained the third dimension of the
metavolcanic-metasedimentary belts, and have contributed to a better understanding of their evolution
and associated mineral deposits (Gupta et al., 1982). Examples of these early detailed surveys include
those by Middleton (1976), Barlow et al. (1976), Gupta and Wadge (1978), and Gupta (1981).
1.2 GRAVITY DATA PROCESSING
1.2.1 Editing
The gravity data used in the compilation of the Bouguer Gravity of Ontario were obtained from the
digital files of Geomatics Canada and the Ontario Geological Survey. A data file containing the station
latitudes, longitudes and Bouguer gravity values in mGal, referred to as the XYZ file, was first converted
to the Lambert Conformal Conic Projection. Using a minimum curvature gridding algorithm, the gravity
data were then gridded to 2000 m cell size for editing purposes (Briggs, 1974). At the location of each
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recorded gravity value in the XYZ file a corresponding interpolated gravity value was extracted from the
gridded file. The difference between the corresponding values were entered into a third data set, known as
the difference data set, which was also gridded to a 2000 m cell size using the minimum curvature
algorithm with a tolerance of fit 0.2 mGal. Using an imaging workstation, colour images of the gridded
difference set and the plot of gravity station locations were displayed concurrently in order to locate
suspect values in the data set. This helped to identify areas of poor fit to the original recorded Bouguer
gravity values. The poor fit resulted from either the coarse station spacing or from errors in the original
recorded Bouguer gravity values.
Areas of large Bouguer gravity difference (>1 mGal) between the original and gridded data sets were
evaluated using Bouguer gravity maps published by the Dominion Observatory of Canada and the
Ontario Geological Survey. These areas were subsequently attributed to unsubstantiated, anomalous
single-point values that create extreme localized gradients, and incorrect station elevations in the
reduction calculations. The editing process resulted in the deletion of a total of 177 questionable stations,
mostly in Hudson Bay, from the XYZ file.
1.2.2 Gridding
a) The Bouguer Gravity Grid:
Due to a large variation in the spacing of the gravity stations (0.3 to 15 km), four test areas were
examined to obtain the optimum gridding parameters. Each selected test area contained a mix of detailed
and regional surveys. In addition, one of the areas also contained shipborne and iceborne gravity stations.
The four test areas were gridded at three different grid intervals (400 m, 1000 m, and 2000 m) using
the minimum curvature algorithm with tolerance of fit to 0.01 mGal. Colour images of the gridded
Bouguer gravity field were displayed on the workstation to check anomaly resolution, grid smoothness,
and to verify that the integrity of the recorded Bouguer gravity values was maintained during the gridding
process. It was found that the 1000 m grid cell size fit the data well within acquisition accuracy, and
exhibited the maximum resolution.
The edited XYZ file of Bouguer gravity data was used to create the final 1000 m x 1000 m cell-size
grid, using the minimum curvature method. This method interpolates each grid cell from a minimum
curvature surface that passes through all the data points. The final 1000 m by 1000 m cell-size (i.e., x cell-
size = 0.012°, y cell-size=0.00925°) Single Master Gravity Grid for Ontario consists of 2303 columns by
1867 rows (Also see Gupta, 1991a). The Bouguer gravity values range from -99.98 mGal to 36 mGal.
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b) The Vertical Gravity Gradient Grid:
The rationale for computing vertical gravity gradient, or a second vertical derivative, is that
anomalously dense rocks occurring near the ground surface produce much stronger gradient effects than
those which lie at great depths (Gupta and Sutcliffe, 1990; Gupta and Grant, 1985; Gupta and Ramani,
1982). However, it should be noted that the amplitudes of the Bouguer gravity anomalies that are caused
by shallow and weak sources may be considerably smaller than those due to larger, deeply buried sources.
The vertical derivative map thus enhances the weak, near-surface anomalies.
The vertical gradient (1000 m by 1000 m cell-size grid) of the Bouguer gravity field was computed by
applying a first vertical derivative filter to the Fourier transform of the Bouguer gravity field grid. A
fourth degree, low-pass, cosine roll-off filter, with roll-off wavelengths commencing at seven km and
ending at five km, was applied to remove the sharp roll-off at the high frequency end of the filter. The
first vertical derivative in the space domain was then obtained by taking the inverse Fourier transform of
the filtered, frequency domain map. The final 1000 m x 1000 m cell-size gravity gradient grid ranges
from -0.0056 mGal/km to 0.00787 mGal/km.
The vertical derivative map (see Gupta, 1992) is used primarily to locate vertical to subvertical
boundaries between rock units of contrasting densities. The zero contour of the vertical derivative
anomalies helps to outline approximate boundaries of the anomalous gravity sources. In general, the
vertical gradient is therefore useful for mapping near-surface structures, such as faults and folds.
1.2.3 Grid Projection
The 1000 m x 1000 m cell-size grid referred to as the Single Master Gravity Grid for Ontario was
compiled using the Lambert Conformal Conic Projection with standard parallels 49° N and 77° N and
central meridian of 92° W. The grid origin was chosen at 0° N (equator) and 92° W. A false Easting of
1,000,000 was applied. The Lambert projection grids were then converted to Latitude-Longitude
projection grids. The projection used the North American Datum 1927 (NAD 27), Canada (Manitoba;
Ontario local datum).
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1.3 DATA SPECIFICATIONS
The Bouguer Gravity grid of Ontario was compiled from approximately 55,639 gravity stations
retrieved from the National Gravity Data Base at the Geological Survey of Canada. The Ontario
Geological Survey has contributed over 23,000 gravity stations to this database. The remaining 32,639
stations were measured by the Dominion Observatory of Canada and its successor the Earth Physics
Branch of Energy, Mines Resources, Canada. The Gravity measurements used in the compilation of this
map are consistent with the standards set forth by the international Gravity Standardization Net 1971
(IGSN71) and the Geodetic Reference System 1967 (GRS67; see Woollard, 1979). Bouguer anomalies
were calculated using a vertical gravity gradient of 0.3086 mGal/m and a crustal density of 2.67 g/cm3.
The data were not terrain-corrected except for those from 40 stations near Sudbury and 54 stations near
Cornwall. In the current data base, the accuracy of the anomalies varies generally from about ±0.5 mGal
for detailed surveys carried out by the Ontario Geological Survey, to about ±2 mGal for regional surveys,
and to about ±5 mGal for shipborne surveys.
2. TOTAL MAGNETIC FIELD AND VERTICAL MAGNETIC GRADIENT OF ONTARIO
2.1 INTRODUCTION
Aeromagnetic surveys have been conducted in Ontario since 1947, when the first survey was
carried out in Northern Ontario by the Gulf Research and Development Company. In 1948, systematic
survey coverage of Ontario was initiated by the Geological Survey of Canada (GSC) and the Ontario
Department of Mines, through federal-provincial agreements. Between 1947 and 1979, data from 32
aeromagnetic surveys were acquired in analog form. Flight directions for these surveys were chosen to
transect the predominant regional structural trends of the underlying rocks. Therefore, most of these
surveys were nominally flown in a north-south direction, with a flight line spacing of 0.5 mile (805 m)
and a mean terrain clearance of 1000 feet (305 m). A few of the surveys were flown in an east-west
direction at a mean terrain clearance of 500 feet (150 m). Between 1984 and 1987, nine digitally recorded
surveys were flown by the GSC in north-south, and to a lesser extent east-west, directions at a flight line
spacing of 1000 m and a mean terrain clearance of 300 m. Aeromagnetic contour maps are available from
the GSC at scales of either 1:63,360 (1 inch to 1 mile) and 1:253,440 (1 inch to 4 miles), or 1:50,000 and
1:250,000.
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In the early 1980s, the GSC initiated a program to create a digital aeromagnetic database by digitizing
existing maps. The digital data set for each survey was gridded at 812.8 m by the GSC and then levelled
at survey boundaries (Dods et al., 1985).
The total magnetic field intensity colour images and computer-enhanced products created from the
GSC grid are useful for regional mapping purposes. However, the 812.8 m grid spacing is too coarse for
these images to be used as an aid in mineral exploration. Therefore, all the aeromagnetic data for Ontario
were recompiled into a contiguous grid of total magnetic field intensity at a smaller grid cell size. A
unique method, utilizing the existing GSC 812.8 m grid data base, was applied to level the 41 surveys
together and create a Single Master Aeromagnetic Grid for the province of Ontario at a chosen uniform
grid cell size 200 x 200 m (Gupta et al., 1989).
2.2 MAGNETIC DATA PROCESSING
2.2.1 Editing
The digitized magnetic survey data were first sorted into contiguous original survey blocks. This was
done by digitizing the boundaries of the various surveys and placing each in the correct location. Both the
digitized and digitally recorded total field data were gridded to a 200 m cell size, using a bi-directional
gridding algorithm. The digitized data were gridded with the Akima spline (Akima, 1970) in both
directions. The digitally recorded data were first interpolated linearly along flight lines, due to the higher
density of data, and then with the Akima spline perpendicular to the flight lines. A contiguous grid for
each of the surveys was thus created. Using an imaging workstation, colour total field grid images and
profiles of the magnetic data along flight lines were displayed concurrently in order to locate and correct
location and digitization errors in the data sets.
To bring the entire data set to a common 300 m altitude, data from the six surveys which were
acquired at a 500-foot (150 m) mean terrain clearance were continued upward to a common altitude of
300 m above ground.
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2.2.2 Levelling
The levelling of the magnetic survey data was carried out using the existing GSC 812.8 m grid as a
reference grid, onto which the unleveled survey grids were draped to produce a levelled grid (Reford et
al., 1990)
The original unleveled magnetic grids (200 m cell size) contain signals arising from three sources: i)
geological sources of short and medium wavelengths, ii) the regional field, and iii) levelling error. Since
the GSC reference grid has been produced from the same data set, its signal contains two of the above
sources, the geological sources of short and medium wavelengths and the regional field.
By subtracting the reference grid from the unleveled grid, the signals due to the geological sources of
medium wavelengths and the regional field were removed. This left the signals due to geological sources
of short wavelengths and the levelling error intact in the unlevelled magnetic grid. A low-pass nonlinear
filter in the flight line direction was applied to remove the signal due to geological sources of short
wavelengths, leaving a grid containing levelling error only. The grid of levelling error was then subtracted
from the original unleveled grid to produce a leveled grid, which contains only the desired signal, that due
to geological sources of short and medium wavelengths and the regional field. The removal of levelling
error between surveys resulted in a seamless fit at mutual boundaries.
2.2.3 Gridding
a) The Total Magnetic Field Grid:
For the digitized magnetic surveys, profile data were extracted from the levelled grid by interpolation,
using a bicubic spline over a 6 by 6 window of 200 m x 200 m grid cells. Similarly, the levelling
corrections for the digitally recorded surveys were extracted from the correction grid and were then
applied to the profile data. Levelled profile data were used to create the final master 200 m x 200 m cell-
size (i.e., x cell-size = 0.002401°, y cell-size = 0.001851°) grid using the minimum curvature method
(Briggs, 1974). This method interpolates each grid cell from a minimum curvature surface that passes
through all the data points. The final 200 m x 200 m cell-size Single Master Aeromagnetic Grid for
Ontario consists of 8455 rows and 9013 columns and the IGRF-corrected magnetic field values range
from -6,232.7 nT to 30,418.2 nT.
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b) The Vertical Magnetic Gradient Grid:
The rationale for computing a vertical magnetic gradient grid is that anomalously magnetic rocks
occurring near the ground surface produce much stronger gradient effects than those that lie at great
depths. However, it should be noted that the amplitudes of the total magnetic field anomalies that are
caused by shallow and weak sources may be considerably smaller than those due to larger, deeply buried
sources. The vertical derivative map thus enhances the weak near-surface anomalies.
The vertical gradient of the magnetic field was computed by applying a first vertical derivative filter
to the Fourier transform of the total magnetic field grid (see Gupta, 1991b). A sixth-degree Butterworth
low-pass filter with a cut-off wavelength of one km was applied to minimize high frequency flight line
and gridding noise. The vertical magnetic gradient map (Gupta, 1991c) in the space domain was then
obtained by taking the inverse Fourier transform of the filtered, frequency domain map. The final 200 m x
200 m cell-size vertical magnetic gradient grid has a range varying from -12.063 nT/m to 48.6401 nT/m.
2.2.4 Grid projection
The 200 m x 200 m grid referred to as the Single Master Aeromagnetic Grid of Ontario was compiled
using the Lambert Conformal Conic Projection with standard parallels of 49° N and 77° N and central
meridian of 92° W. The grid origin was chosen at 0° N (equator) and 92° W. A false Easting of 1,000,000
was applied. The projection used the North American Datum 1927 (NAD 27), Canada (Manitoba;
Ontario local datum).
3. ADDITIONAL DATA PROCESSING
3.1 INTRODUCTION
The three geophysical data sets were converted to flat ASCII and Geosoft formats by Controlled
Geophysics. To preserve the integrity of the data, no data values were deleted or modified in any way.
The parameters in the data set and the grids are described in Appendix A.
3.2 GRAVITY DATA
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The gravity station data have been provided in Geosoft oasis montaj, flat ASCII text, Microsoft
Access™, and Microsoft Excel™ formats. The Bouguer gravity and vertical gravity gradient grids were
prepared in Geosoft binary .GRD and ASCII .GXF formats.
3.3 DIGITIZED MAGNETIC DATA
The ERLIS # 1 data CD-ROM contains the digitized magnetic profile data, organized as one file per
NTS sheet. There are 54,395 survey line segments in the provincial digitized magnetic database. Line
numbers had been assigned arbitrarily during digitizing. A specialized routine was applied to link lines
that were broken during the original map digitizing.
Each new line comprised of linked segments was assigned a unique line number derived from a
combination of the NTS sheet value and a line number counter. The new line numbers have the format
XXYYYYY, where XX is the NTS sheet number and YYYYY is the linked line number counter.
Each of the eleven NTS data files was linked individually, and then all NTS sheets were combined
into a single, master profile data file. Note that crossing survey lines made it impossible to achieve perfect
linking. A number of digitized points that had been assigned to the wrong line number in the original data
archive, were not corrected.
The profile data set was prepared in Geosoft oasis montaj and flat ASCII text formats. The total magnetic
field and vertical magnetic gradient grids were prepared in Geosoft binary and ASCII GXF formats.
3.4 DIGITALLY RECORDED MAGNETIC DATA
The ERLIS # 1 CD-ROM contains magnetic profile data organized as one file for each of nine
detailed survey areas. Line numbers are re-used from one area to another. To store all nine data sets in a
single file, each area was assigned a two-digit code, which was added to the existing line number as a
prefix. The new line numbers have the format XXYYYYY, where XX is the two-digit code number and
YYYYY is the original line number. The two-digit codes are:
BORDER .............11 HURON ...............14 ST CLAIR............17
ERIE.....................12 ONTARIO............15 SUPERIOR ..........18
GEORGIAN.........13 SIMCOE ..............16 WATERLOO .......19
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The profile data set was prepared in Geosoft oasis montaj and flat ASCII text formats.
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REFERENCES
Akima, H., 1970. A new method of interpolation and smooth curve fitting based on local procedures;
Journal of the Association for Computing Machinery, v.17, no.4, p.589-602.
Barlow, R.B., Gupta, V.K. and Wadge, D.R., 1976. Bouguer gravity map, Birch-Uchi-Confederation
Lakes area, District of Kenora (Patricia Portion); Ontario Division of Mines, Preliminary Map
P.1186, scale 1:126,720.
Briggs, I.C., 1974. Machine contouring using minimum curvature; Geophysics, v.39, no.1, p.39-48.
Dods, S.D., Teskey, D.J. and Hood, P.J., 1985. The new series of 1:1,000,000 scale magnetic anomaly
maps of the Geological Survey of Canada: compilation techniques and interpretation; in The Utility
of Regional Gravity and Magnetic Anomaly Maps, Society of Exploration Geophysicists, p.69-87.
Gupta, V.K. and Wadge, D.R., 1978. Bouguer gravity and generalized geologic map, Red Lake area,
District of Kenora (Patricia Portion); Ontario Geological Survey, Preliminary Map p.1248, scale
1:00,000.
Gupta, V.K. and Ramani, N., 1980. Some aspects of regional-residual separation of gravity anomalies in
a Precambrian terrain; Geophysics, v.45, p.1412-1426.
Gupta V.K., 1981. Bouguer gravity and generalized geological map, Gogama-Gowganda area, districts
of Sudbury and Timiskaming; Ontario Geological Survey, Preliminary Map P.2481, scale 1:100,000.
Gupta, V.K. and Ramani, N., 1982. Optimum second vertical derivatives in geological mapping and
mineral exploration; Geophysics, v.47, p.1706-1715.
Gupta, V.K., Thurston, P.C. and Dusanowskyj, T.H., 1982. Constraints upon models of greenstone belt
evolution by gravity modelling, Birch-Uchi greenstone belt, northern Ontario; Precambrian Research,
v.16, p.233-255.
Gupta, V.K. and Grant, F.S., 1985. Mineral-exploration aspects of gravity and aeromagnetic surveys in
the Sudbury-Cobalt area, Ontario; in The Utility of Regional Gravity and Aeromagnetic Anomaly
Maps, Society of Exploration Geophysicists, Special Volume, p.392-411.
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Gupta, V., Paterson, N., Reford, S., Kwan, K., Hatch, D. and MacLeod, I., 1989. Single master
aeromagnetic grid and magnetic colour maps for the province of Ontario; in Summary of Field Work
and Other Activities 1989, Ontario Geological Survey, Miscellaneous Paper 146, p.244-250.
Gupta, V.K. and Sutcliffe, R.H., 1990. Mafic-ultramatic intrusives and their gravity field: Lac des lles
area, northern Ontario; Geological Society of America Bulletin, v.102, p.1417-1483.
Gupta V.K., 1991a. Bouguer gravity of Ontario; Ontario Geological Survey, Maps 2592 to 2595, scale
1:1,000,000.
Gupta, V.K., 1991b. Shaded image of total magnetic field of Ontario; Ontario Geological Survey, Maps
2584 to 2587, scale 1:1,000,000.
Gupta, V.K., 1991c. Vertical magnetic gradient of Ontario; Ontario Geological Survey, Maps 2588 to
2591, scale 1:1,000,000.
Gupta, V.K., 1992. Shaded image of vertical gravity gradient of Ontario, Ontario Geological Survey,
Maps 2596 to 2599, scale 1:1 000,000.
Innes, M.J.S., 1960. Gravity and isostasy in northern Ontario and Manitoba; Publication of the Dominion
Observatory, v.21, no.6, p.263-338.
Middleton, R.S., 1976. Gravity survey of geological structures in the Timmins and Matheson area,
districts of Cochrane, Timiskaming and Sudbury; Ontario Division of Mines, Report 135, 45p.
Reford, S.W., Gupta, V.K., Paterson, N.R., Kwan, K.C.H. and MacLeod, I.N., 1990. The Ontario master
aeromagnetic grid: a blueprint for detailed compilation of magnetic data on a regional scale; in
Expanded Abstracts, Society of Exploration Geophysicists, 60th Annual International Meeting, San
Francisco, v.1, p.617-619.
Woollard, G.P., 1979. The new gravity system-changes in international gravity base values and anomaly
values; Geophysics, v.44, p.1352-1366.
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CREDITS
Project management and supervision by V.K. Gupta, Ontario Geological Survey.
Processing and grid generation was carried out, under contract, by Paterson, Grant and Watson, Ltd.,
Toronto.
Conversion of all data to Geosoft format and linking of fragmented digitized magnetic flight line
segments was carried out, under contract, by CGI Controlled Geophysics Inc., Thornhill, Ontario.
SOURCES OF INFORMATION
Digital gravity data were provided by Geodetic Survey Division, Geomatics Canada, Natural Resources
Canada and Ontario Geological Survey.
Digitally recorded and digitized aeromagnetic survey data, including the 812.8 m aeromagnetic grid for
Ontario, were provided by the Geological Survey of Canada.
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APPENDIX A - CONTENTS OF PROFILE AND GRID DATA FILES
Note: i) ERLIS CD-ROM # 1035 contains gravity and magnetic point data (XYZ) in ASCII .TXT format
and grid data in ASCII .GXF format. In addition, gravity point data (XYZ) are also provided in
Microsoft ACCESS™ and EXCEL™ formats.
ii) ERLIS CD-ROM # 1036 contains gravity and magnetic point data (XYZ) in Geosoft oasis
montaj .GDB format and grid data in Geosoft .GRD format.
ONTARIO BOUGUER GRAVITY STATION XYZ VALUES
(ASCII format file name: ONGRAVTY.TXT)
(Geosoft oasis montaj format file name: ONGRAVTY.GDB)
Parameter Unit Description
LX metres Lambert Conformal Easting
LY metres Lambert Conformal Northing
LAT degrees Latitude
LONG degrees Longitude
BOUGUER mgal Bouguer Gravity
ONTARIO AEROMAGNETIC XYZ VALUES DIGITIZED FROM CONTOUR MAPS
(ASCII format file name: ONDTZMAG.TXT)
(Geosoft oasis montaj format file name: ONDTZMAG.GDB)
Parameter Unit Description
LINE na Line Number
LX metres Lambert Conformal Easting
LY metres Lambert Conformal Northing
LAT degrees Latitude
LONG degrees Longitude
EDITMAG nT Edited Digitized Total Magnetic Field
LEVMAG nT Levelled Digitized Total Magnetic Field
RAWMAG nT Original Digitized Total Magnetic Field
OLDLINE na Original Digitizing Line Number
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ONTARIO AEROMAGNETIC XYZ DATA RECORDED DIGITALLY OVER NINE SURVEY
AREAS
(ASCII format file name: ONDIGMAG.TXT)
(Geosoft oasis montaj format file name: ONDIGMAG.GDB)
Parameter Unit Description
LINE na Line Number
LX metres Lambert Conformal Easting
LY metres Lambert Conformal Northing
LAT degrees Latitude
LONG degrees Longitude
RAWMAG nT Original Magnetic Total Field
LEVMAG nT Final Levelled Magnetic Total Field
OLDLINE na Original Contractor Line Number
The line numbers (LINE) have the format XXYYYYY, where XX is a two-digit code for each
survey area and YYYYY is the original line number (OLDLINE). The two-digit codes are:
BORDER .............11 HURON ...............14 ST CLAIR............17
ERIE.....................12 ONTARIO............15 SUPERIOR ..........18
GEORGIAN.........13 SIMCOE ..............16 WATERLOO .......19
ONTARIO SINGLE MASTER MAGNETIC AND GRAVITY GRIDS
Grids are named so that the first two characters represent the name of the survey area followed by a
standard set of grid type codes, e.g., ONGRV1VD is the first vertical derivative of gravity in Ontario. All
Geosoft binary grids are provided in Real*4 precision. All ASCII grids are Geosoft GXF format.
ONGRAVTY.GXF mgal Bouguer gravity grid (1000m x 1000 m) Geosoft ASCII format
ONGRV1VD.GXF mgal/km Vertical gravity gradient grid (1000 m x 1000 m) Geosoft ASCII format
ONMAGONL.GXF nT Total magnetic field grid (200 m x 200 m) Geosoft ASCII format
ONMAG1VD.GXF nT/m Vertical magnetic gradient grid (200 m x 200 m) Geosoft ASCII format
ONGRAVTY.GRD mgal Bouguer gravity grid (1000m x 1000 m) Geosoft binary format
ONGRV1VD.GRD mgal/km Vertical gravity gradient grid (1000 m x 1000 m) Geosoft binary format
ONMAGONL.GRD nT Total magnetic field grid (200 m x 200 m) Geosoft binary format
ONMAG1VD.GRD nT/m Vertical magnetic gradient grid (200 m x 200 m) Geosoft binary format
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