We'll download the ship from a public source and isolate it.
ship = imread('https://e3.365dm.com/21/03/768x432/skynews-ever-given-suez-canal_5320248.jpg?20210327160438'); imshow(ship)
I could probably come up with a way to automatically segment the ship. But I only need to do it once, so I'll just draw the polygon and mask it.
if false % Draw polygon on border of ship p = drawpolygon; else load('p.mat', "p") imshow("eg.png") end
Mask the ship based on the polygon and apply that to the original image.
shipmask = poly2mask(p.Position(:, 1), p.Position(:, 2), height(ship), width(ship)); shipmask = imerode(shipmask, strel("disk", 2)); figure tiledlayout(2, 1, TileSpacing="compact", Padding="tight") nexttile imshow(shipmask) nexttile ship = ship.*cast(shipmask, class(ship)); imshow(ship)
Get the orientation from the ship mask and apply it to the original image.
shipprops = regionprops(shipmask, "Orientation") shiphorizontal = imrotate(ship, -shipprops.Orientation); figure imshow(shiphorizontal)
shipprops = struct with fields: Orientation: 19.5882
Crop to just the ship
shipbbox = regionprops(logical(shiphorizontal(:, :, 1)), "BoundingBox") shipthumb = imcrop(shiphorizontal, shipbbox.BoundingBox); imshow(shipthumb)
shipbbox = struct with fields: BoundingBox: [181.5000 320.5000 427 67]
We'll download the satellite imagery from USGS for the region we care about. I'll interactively pick the regional limits for the area I care about.
if false % Interactively select limits w = webmap [latlim, lonlim] = wmlimits(w) else latlim = [42.2966508472710 42.3053796253471] lonlim = [-71.3800315706306 -71.3639383165411] end
latlim = 42.2967 42.3054 lonlim = -71.3800 -71.3639
Find the ortho layer and download it for this region.
ortho = wmsfind('usgsimageryonly', SearchField='serverurl'); [lakeside, lakesidereference] = wmsread(ortho, latlim=latlim, lonlim=lonlim); figure geoshow(lakeside, lakesidereference) axis tight
I want to put the ship in the little lake between Rt 9 and the railroad track immediately adjacent to MathWorks' Lakeside campus. It looks like a ship orientation of about will work well.
shipn75 = imrotate(shipthumb, -75,'nearest'); figure imshow(shipn75);
We'll work in meters and need the dimensions of the ship. A quick web search says the length is 399.94m and the beam is 58.8m.
shiplength = 399.94; shipbeam = 58.8;
At the same time will check the ratio of length to width with that of our image.
props = regionprops(logical(shipn75(:,:,1)), "MajorAxisLength", "MinorAxisLength"); image_shipratio = props.MajorAxisLength/props.MinorAxisLength actual_shipratio = shiplength/shipbeam
image_shipratio = 6.7436 actual_shipratio = 6.8017
I'm happy with that(!) especially considering I just approximated the ship with a drawn polygon.
Now we need to figure out the size of this rotated image in meters. Assuming the ship was a perfect rectangle, we can calculate this size using some trigonometry. It's been awhile since I've done this so I broke out a trusty envelope and tried to remember soh-cah-toa. The equations for image dimensions are:
imgheightm = shipbeam*sind(15)+shiplength*cosd(15) imgwidthm = shiplength*sind(15)+shipbeam*cosd(15)
imgheightm = 401.5309 imgwidthm = 160.3085
Finally, we need to convert the background of the image to NaN so when we plot it shows as transparent. This is where all channels are 0.
shipoverlay = im2double(shipn75); shipoverlay(repmat(all(~shipoverlay, 3), [1 1 3])) = NaN;
We need to project the satellite imagery from geographic coordinates (lat/lon) to cartesian coordinates (x/y in meters) to overlay the boat on it. We'll use the Mass State Plane coordinate reference system which is optimized for Massachusetts. A quick web search tells me that the code for this coordinate system is 26986.
p = projcrs(26986) [latgrid, longrid] = geographicGrid(lakesidereference); [xgrid, ygrid] = projfwd(p, latgrid, longrid); figure mapshow(xgrid,ygrid,lakeside) axis tight
p = projcrs with properties: Name: "NAD83 / Massachusetts Mainland" GeographicCRS: [1×1 geocrs] ProjectionMethod: "Lambert Conic Conformal (2SP)" LengthUnit: "meter" ProjectionParameters: [1×1 map.crs.ProjectionParameters]
I'll use a datatip to find the upper left corner of where the boat should go. The live editor gives me the below code to reproduce this.
ax = gca; chart = ax.Children(1); datatip(chart, 210495, 894667);
Since the map is now in projected coordinates, the x/y values represent meters. We can create map reference cells that describe the pixel size of the boat image and location which then makes plotting it easy.
xpt = 210495; ypt = 894667; mr = maprefcells([xpt xpt+imgwidthm], [ypt-imgheightm ypt], size(shipoverlay, [1 2]), ColumnsStartFrom="north")
mr = MapCellsReference with properties: XWorldLimits: [210495 210655.308527484] YWorldLimits: [894265.469065182 894667] RasterSize: [432 177] RasterInterpretation: 'cells' ColumnsStartFrom: 'north' RowsStartFrom: 'west' CellExtentInWorldX: 0.905697895390393 CellExtentInWorldY: 0.929469756523354 RasterExtentInWorldX: 160.3085274841 RasterExtentInWorldY: 401.530934818089 XIntrinsicLimits: [0.5 177.5] YIntrinsicLimits: [0.5 432.5] TransformationType: 'rectilinear' CoordinateSystemType: 'planar' ProjectedCRS: 
And now we can finally plot the Lakeside image with the Ever Given overlaid on top to scale.
figure hold on mapshow(xgrid, ygrid, lakeside) s = mapshow(zeros(size(shipoverlay, [1 2])), mr, DisplayType="surface", CData=shipoverlay); axis tight xticks() yticks()
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