Magnetic Anomaly Maps

The following are key functions related to magnetic anomaly maps.

Load Map Data

MagNav.get_map — Function

get_map(map_file::String   = namad,
        map_field::Symbol  = :map_data;
        map_info::String   = splitpath(map_file)[end],
        map_units::Symbol  = :rad,
        file_units::Symbol = :deg,
        flip_map::Bool     = false)

Get map data from saved HDF5 or MAT file or folder containing CSV files. Maps are typically saved in :deg units, while :rad is used internally.

Arguments:

map_file: path/name of map data HDF5 or MAT file (.h5 or .mat extension required) or folder containing CSV files
map_field: (optional) struct field within MAT file to use, not relevant for CSV or HDF5 file
map_info: (optional) map information, only used if not in map_file
map_units: (optional) map xx/yy units to use in map_map {:rad,:deg}
file_units: (optional) map xx/yy units used in map_file {:rad,:deg}
flip_map: (optional) if true, vertically flip data from map file (possibly useful for CSV map)

Returns:

map_map: Map magnetic anomaly map struct

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get_map(map_name::Symbol, df_map::DataFrame,
        map_field::Symbol  = :map_data;
        map_info::String   = "$map_name",
        map_units::Symbol  = :rad,
        file_units::Symbol = :deg,
        flip_map::Bool     = false)

Get map data from saved HDF5 or MAT file or folder containing CSV files via DataFrame lookup. Maps are typically saved in :deg units, while :rad is used internally.

Arguments:

map_name: name of magnetic anomaly map
df_map: lookup table (DataFrame) of map data HDF5 and/or MAT files and/or folder containing CSV files

Field	Type	Description
`map_name`	`Symbol`	name of magnetic anomaly map
`map_file`	`String`	path/name of map data HDF5 or MAT file (`.h5` or `.mat` extension required) or folder containing CSV files

map_info: (optional) map information, only used if not in map_file
map_field: (optional) struct field within MAT file to use, not relevant for CSV or HDF5 file
map_units: (optional) map xx/yy units to use in map_map {:rad,:deg}
file_units: (optional) map xx/yy units used in files within df_map {:rad,:deg}
flip_map: (optional) if true, vertically flip data from map file (possibly useful for CSV map)

Returns:

map_map: Map magnetic anomaly map struct

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Built-in map data

MagNav.ottawa_area_maps — Function

ottawa_area_maps(f::Union{String,Symbol} = "")

Magnetic anomaly maps near Ottawa, Ontario, Canada, contains:

Eastern_395.h5: Eastern Ontario at 395 m HAE
Eastern_drape.h5: Eastern Ontario on drape
Renfrew_395.h5: Renfrew at 395 m HAE
Renfrew_555.h5: Renfrew at 555 m HAE
Renfrew_drape.h5: Renfrew on drape
HighAlt_5181.h5: High Altitude mini-survey (within Renfrew) at 5181 m HAE
Perth_800.h5: Perth mini-survey (within Eastern Ontario) at 800 m HAE

NOTE: Missing map data within each map has been filled in (using k-nearest neighbors) so that the maps are fully filled. Care must be taken to not navigate in the filled-in areas, as this is not real data and only done for more accurate and consistent upward continuation of the maps. Use the map_check function with the desired map and flight path data to check if the map may be used without navigating into filled-in (artificial) areas.

Arguments:

f: (optional) name of data file (.h5 extension optional)

Returns:

p: path of folder or f data file

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MagNav.namad — Constant

const namad

North American Magnetic Anomaly Database (NAMAD). Compiled from marine and airborne magnetic measurements by the U.S. Geological Survey (USGS), Geological Survey of Canada (GSC), and Consejo de Recursos Minerales of Mexico (CRM). Reference: https://www.usgs.gov/maps/magnetic-anomaly-map-north-america

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MagNav.emag2 — Constant

const emag2

Earth Magnetic Anomaly Grid with 2 arcminute resolution (EMAG2). Compiled from satellite, marine, and airborne magnetic measurements. Reference: https://www.ncei.noaa.gov/products/earth-magnetic-model-anomaly-grid-2

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MagNav.emm720 — Constant

const emm720

Enhanced Magnetic Model (EMM). Compiled from satellite, marine, airborne, and ground magnetic measurements. Expands the scalar crustal field up to spherical harmonic degree and order 720, providing a vector of the magnetic field with approximately 15 arcminute resolution. Underlying crustal field model derived from Earth Magnetic Anomaly Grid with 2-arc-minute resolution (EMAG2). Reference: https://www.ncei.noaa.gov/products/enhanced-magnetic-model

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Upward Continue Map

MagNav.upward_fft — Function

upward_fft(map_map::Matrix, dx, dy, dz; expand::Bool = true, α = 0)

Upward continuation of a potential field (i.e., magnetic anomaly field) map. Uses the Fast Fourier Transform (FFT) to convert the map to the frequency domain, applies an upward continuation filter, and uses the inverse FFT to convert the map back to the spatial domain. Optionally expands the map temporarily with periodic padding. Downward continuation may be performed to a limited degree as well, but be careful, as this is generally unstable and amplifies high frequencies (i.e., noise).

Reference: Blakely, Potential Theory in Gravity and Magnetic Applications, 2009, Chapter 12 & Appendix B (pg. 315-317 & 402).

Arguments:

map_map: ny x nx 2D gridded map data
dx: x-direction map step size [m]
dy: y-direction map step size [m]
dz: nz z-direction upward/downward continuation distance(s) [m]
expand: (optional) if true, expand map temporarily to reduce edge effects
α: (optional) regularization parameter for downward continuation

Returns:

map_map: ny x nx (x nz) 2D or 3D gridded map data, upward/downward continued

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upward_fft(map_map::Map, alt; expand::Bool = true, α = 0)

Reference: Blakely, Potential Theory in Gravity and Magnetic Applications, 2009, Chapter 12 & Appendix B (pg. 315-317 & 402).

Arguments:

map_map: Map magnetic anomaly map struct
alt: target upward continuation altitude(s) [m]
expand: (optional) if true, expand map temporarily to reduce edge effects
α: (optional) regularization parameter for downward continuation

Returns:

map_map: Map magnetic anomaly map struct, upward/downward continued (MapS with alt vector => MapS3D)

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Interpolate Map

MagNav.map_interpolate — Function

map_interpolate(map_map::AbstractArray{T},
                map_xx::AbstractVector{T},
                map_yy::AbstractVector{T},
                type::Symbol = :cubic,
                map_alt::AbstractVector{T} = T[0]) where T

Create map interpolation function, equivalent of griddedInterpolant in MATLAB.

Arguments:

map_map: ny x nx (x nz) 2D or 3D gridded map data
map_xx: nx map x-direction (longitude) coordinates
map_yy: ny map y-direction (latitude) coordinates
type: (optional) type of interpolation {:linear,:quad,:cubic}
map_alt: (optional) map altitude levels

Returns:

itp_map: map interpolation function (f(yy,xx) or (f(yy,xx,alt))

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map_interpolate(mapS::Union{MapS,MapSd,MapS3D}, type::Symbol = :cubic;
                return_vert_deriv::Bool = false)

Create map interpolation function, equivalent of griddedInterpolant in MATLAB. Optionally return vertical derivative map interpolation function, which is calculated using finite differences between map and 1 m upward continued map.

Arguments:

mapS: MapS, MapSd, or MapS3D scalar magnetic anomaly map struct
type: (optional) type of interpolation {:linear,:quad,:cubic}
return_vert_deriv: (optional) if true, also return der_map

Returns:

itp_map: map interpolation function (f(yy,xx) or (f(yy,xx,alt))
der_map: if return_vert_deriv = true, vertical derivative map interpolation function (f(yy,xx) or (f(yy,xx,alt))

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map_interpolate(mapV::MapV, dim::Symbol = :X, type::Symbol = :cubic)

Create map interpolation function, equivalent of griddedInterpolant in MATLAB.

Arguments:

mapV: MapV vector magnetic anomaly map struct
dim: map dimension to interpolate {:X,:Y,:Z}
type: (optional) type of interpolation {:linear,:quad,:cubic}

Returns:

itp_map: map interpolation function (f(yy,xx))

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Plot Map

MagNav.plot_map — Function

plot_map(map_map::Matrix,
         map_xx::Vector       = [],
         map_yy::Vector       = [];
         clims::Tuple         = (),
         dpi::Int             = 200,
         margin::Int          = 2,
         Nmax::Int            = 6*dpi,
         legend::Bool         = true,
         axis::Bool           = true,
         map_color::Symbol    = :usgs,
         bg_color::Symbol     = :white,
         map_units::Symbol    = :rad,
         plot_units::Symbol   = :deg,
         b_e::AbstractBackend = gr())

Plot map.

Arguments:

map_map: ny x nx 2D gridded map data
map_xx: nx map x-direction (longitude) coordinates [rad] or [deg]
map_yy: ny map y-direction (latitude) coordinates [rad] or [deg]
clims: (optional) length-2 colorbar limits (cmin,cmax)
dpi: (optional) dots per inch (image resolution)
margin: (optional) margin around plot [mm]
Nmax: (optional) maximum number of data points plotted (per axis)
legend: (optional) if true, show legend
axis: (optional) if true, show axes
map_color: (optional) filled contour color scheme {:usgs,:gray,:gray1,:gray2,:plasma,:magma}
bg_color: (optional) background color
map_units: (optional) map xx/yy units {:rad,:deg}
plot_units: (optional) plot xx/yy units {:rad,:deg,:m}
b_e: (optional) plotting backend

Returns:

p1: plot of map

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plot_map(map_map::Map;
         use_mask::Bool       = true,
         clims::Tuple         = (),
         dpi::Int             = 200,
         margin::Int          = 2,
         Nmax::Int            = 6*dpi,
         legend::Bool         = true,
         axis::Bool           = true,
         map_color::Symbol    = :usgs,
         bg_color::Symbol     = :white,
         map_units::Symbol    = :rad,
         plot_units::Symbol   = :deg,
         b_e::AbstractBackend = gr())

Plot map.

Arguments:

map_map: Map magnetic anomaly map struct
use_mask: (optional) if true, apply map_map mask to map
clims: (optional) length-2 colorbar limits (cmin,cmax)
dpi: (optional) dots per inch (image resolution)
margin: (optional) margin around plot [mm]
Nmax: (optional) maximum number of data points plotted (per axis)
legend: (optional) if true, show legend
axis: (optional) if true, show axes
map_color: (optional) filled contour color scheme {:usgs,:gray,:gray1,:gray2,:plasma,:magma}
bg_color: (optional) background color
map_units: (optional) map xx/yy units {:rad,:deg}
plot_units: (optional) plot xx/yy units {:rad,:deg,:m}
b_e: (optional) plotting backend

Returns:

p1: plot of map (if map_map isa MapV, mapX)
p2: if map_map isa MapV, mapY
p3: if map_map isa MapV, mapZ

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