How to find points inside Singapore?

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Shunan Sheng
Shunan Sheng on 13 Jan 2022
Commented: Adam Danz on 18 Jan 2022
I have generated a set of geographical points with latitude and longtitude, I want to pick up the points residing inside the border of Singapore. There are similar answers in https://www.mathworks.com/matlabcentral/answers/311766-how-to-mask-data-points-outside-a-border-using-geoshow and https://www.mathworks.com/matlabcentral/answers/444653-how-can-i-remove-the-region-outside-of-the-vector-data-when-i-combine-the-vector-and-raster-data-on. However. I failde to replicate the code as it seems that shaperead does not contain information about Singapore . Where can I find the border information needed?
S = shaperead('landareas', 'UseGeoCoords', true,...
'Selector',{@(name) strcmp(name,'Singapore'), 'Name'}); % the original code is "Australia"
  2 Comments
Shunan Sheng
Shunan Sheng on 14 Jan 2022
Thank you so much for the reply! Do you know places where I can find precise border for small countries like Singapore?

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Accepted Answer

Cris LaPierre
Cris LaPierre on 13 Jan 2022
Edited: Cris LaPierre on 13 Jan 2022
You are correct, landareas does not contain a shape with the name 'Singapore'.
landareas = shaperead('landareas.shp','UseGeoCoords',true);
figure;
axesm('miller','MapLatLimit',[-1 4],'MapLonLimit',[100 108]);
geoshow(landareas)
find(strcmp({landareas.Name},'Singapore'))
ans = 1×0 empty double row vector
If you want to capture the polygon here that may represent Singapore in landareas.shp, you will have to find it manually.
figure
geoshow(landareas(372));
You can extract the associated lat and lon values, and use inpolygon to search for points that are within the shape.
singLat = landareas(372).Lat;
singLon = landareas(372).Lon;
  4 Comments
Sean de Wolski
Sean de Wolski on 14 Jan 2022
inpolygon will not work unless you project the coordinates from lat/lon to a cartesian coordinate system. In R2021b, there's a new class geopolyshape that has an isinterior method that makes this much easier than it used to be.

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More Answers (1)

Adam Danz
Adam Danz on 14 Jan 2022
Edited: Adam Danz on 14 Jan 2022
> Do you know places where I can find precise border for small countries like Singapore?
Search the internet for shp files containing information about Singapore. I briefly searched data.gov.sg and found 294 shp files on Singapore from census data, municiple planning areas, and lots of different maps.
Here's a demo using the 2010 Singapore Region Census.
  1. Go to the 2010 census link above, download the zip file; unzip all files.
  2. Add the folder containing Region_Census2010.shp to your MATLAB path using addpath(...)
  3. Run the code below to see the map produced below. Note, these data break Singapore into Central, West, North, North-East, and East regions but is also includes at least some of the island borders.
  4. See notes in code below for more info.
First let's look at the shape info
info = shapeinfo('Region_Census2010.shp')
info =
struct with fields:
Filename: [3×108 char]
ShapeType: 'Polygon'
BoundingBox: [2×2 double]
NumFeatures: 5
Attributes: [8×1 struct]
CoordinateReferenceSystem: [1×1 projcrs]
info.CoordinateReferenceSystem
ans =
projcrs with properties:
Name: "SVY21"
GeographicCRS: [1×1 geocrs]
ProjectionMethod: "Transverse Mercator"
LengthUnit: "meter"
ProjectionParameters: [1×1 map.crs.ProjectionParameters]
The ProjectionMethod is "Transverse Mercator" which is "tranmerc" in Matlab if you choose to use map axes (eg, axesm('tranmerc')).
The coordinate reference system is not a geocrs object so this dataset does not use geographical coordinates (lat/lon) as described here. If you need Lat/Lon you'll either need to find a different sph file or perhaps these values can be converted.
Plot singapore
% read shp file
% Note that S is a 5x1 structure: 1 for each region of singapore
S = shaperead('Region_Census2010.shp','UseGeoCoords',true)
% Plot regions
axes()
hold on
for i = 1:numel(S)
plot(S(i).Lon, S(i).Lat)
end
axis equal
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