projfwd

Project latitude-longitude coordinates to x-y map coordinates

Syntax

[x,y] = projfwd(proj,lat,lon)

Description

[x,y] = projfwd(proj,lat,lon) transforms the latitude-longitude coordinates specified by lat and lon to the x and y map coordinates in the projected coordinate reference system specified by proj. Specify proj using a projcrs object (since R2020b), a map projection structure, or a GeoTIFF information structure.

Examples

Project Latitude-Longitude Coordinates to x-y

Project latitude-longitude coordinates to x-y coordinates by specifying a map projection. Then, display the projected coordinates on a map.

To do this, first specify the latitude and longitude coordinates of landmarks in Boston. Specify the coordinates in the NAD83 geographic CRS.

lat = [42.3604 42.3691 42.3469 42.3480 42.3612];
lon = [-71.0580 -71.0710 -71.0623 -71.0968 -71.0941];

Then, import a GeoTIFF image of Boston as an array and a map cells reference object. Get information about the map projection by querying the ProjectedCRS property of the reference object. Verify that the geographic CRS underlying the projected CRS is NAD83.

[A,R] = readgeoraster("boston.tif");
proj = R.ProjectedCRS;
proj.GeographicCRS.Name

ans = 
"NAD83"

Project the latitude-longitude coordinates to x-y coordinates using the same projected CRS as the GeoTIFF image.

[x,y] = projfwd(proj,lat,lon);

Display the GeoTIFF image and the projected coordinates on the same map. Change the marker symbol and color of the coordinates so they are more visible. Then, add axis labels.

mapshow(A,R)
mapshow(x,y,DisplayType="point",Marker="o", ...
    MarkerFaceColor="y",MarkerEdgeColor="none")
xlabel("x (survey feet)")
ylabel("y (survey feet)")

Input Arguments

proj — Map projection

projcrs object | scalar map projection structure | scalar GeoTIFF information structure

Map projection, specified as a projcrs object (since R2020b), a scalar map projection structure (mstruct), or a GeoTIFF information structure. For more information about map projection structures, see defaultm. For more information about GeoTIFF information structures, see geotiffinfo.

Data Types: struct

lat — Geodetic latitudes

scalar value | vector | matrix | N-D array

Geodetic latitudes, specified as a scalar value, vector, matrix, or N-D array, in units of degrees. The size of lat and lon must match.

Data Types: single | double

lon — Geodetic longitudes

scalar value | vector | matrix | N-D array

Geodetic longitudes, specified as a scalar value, vector, matrix, or N-D array, in units of degrees. The size of lat and lon must match.

Data Types: single | double

Output Arguments

x — Projected x-coordinates

scalar value | vector | matrix | N-D array

Projected x-coordinates, returned as a scalar value, vector, matrix, or N-D array.

y — Projected y-coordinates

scalar value | vector | matrix | N-D array

Projected y-coordinates, returned as a scalar value, vector, matrix, or N-D array.

Tips

If the geographic CRS of lat and lon does not match the geographic CRS of proj, then the values of x and y may be inaccurate. When proj is a projcrs object, you can find its geographic CRS by querying its GeographicCRS property. For example, this code shows how to create a projcrs object from EPSG code 32610 and find the associated geographic CRS.

proj = projcrs(32610);
proj.GeographicCRS.Name

ans = 

    "WGS 84"

Version History

Introduced before R2006a

R2022b: projfwd shows improved performance on Windows with repeated use of projection

The projfwd and projinv functions show improved performance on Windows^® when used multiple times in the same MATLAB^® session with the same projcrs object or map projection structure as input.

For example, this code is about 1.67 times faster than in the previous release. The code:

• Gets the grid coordinates for a DTED file

• Projects and unprojects the coordinates using a projcrs object

• Gets the grid coordinates for a different DTED file

• Measures the time required to project and unproject the coordinates using the same projcrs object

function timingTest
    info1 = georasterinfo("n39_w106_3arc_v2.dt1");
    R1 = info1.RasterReference;
    [latGrid1,lonGrid1] = geographicGrid(R1);

    p = projcrs(4088);
    [xfwd1,yfwd1] = projfwd(p,latGrid1,lonGrid1);
    [latinv1,loninv1] = projinv(p,xfwd1,yfwd1);

    info2 = georasterinfo("n40_w106_3arc_v2.dt1");
    R2 = info2.RasterReference;
    [latGrid2,lonGrid2] = geographicGrid(R2);

    tic
    [xfwd2,yfwd2] = projfwd(p,latGrid2,lonGrid2);
    [latinv2,loninv2] = projinv(p,xfwd2,yfwd2);
    toc
end

The approximate execution times are:

• R2022a: 0.70 s

• R2022b: 0.42 s

This code was timed on a Windows 10 Intel^® Xeon^® CPU W-2133 @ 3.6 GHz test system, by calling the function timingTest.

R2020b: Project coordinates using projcrs objects and additional map projection structures

Project coordinates by specifying a projcrs object. A projcrs object enables you to transform coordinates using projection methods not available in earlier releases, such as Hotine Oblique Mercator.

Additionally, you can project coordinates by specifying a map projection structure with any valid mapprojection field. For a list of mapprojection field options, use the maplist or maps function.