Resample points that make up a line

My dataset consists of lines that are defined by "group_id" and "counter." Each group_id is one line, and the counter is the order of the points to build the line. x & y fields are the coordinates in UTM meters.

How can I resample the lines at a specific spacing (e.g., 200 meters)? I don't have the mapping toolbox.

Dataset example attached

Answer 1

[newbie9]

I solved this by [for each group_id] getting the distance between each counter_id. Then if the distance was less than the threshold, I used linspace to resample the points to make up the line. The distance check/resampling is in the function at the bottom of the script. The figure shows that the spacing has been resampled as expected.

datain = readtable('example_data.txt');
interval = 200;

% pre-allocate empty table
colnames = datain.Properties.VariableNames;
temptable = array2table(NaN(300*1000,length(colnames)));
temptable.Properties.VariableNames = colnames;
rownum = 1;

% descritize 
for ii = 202 %min(datain.group_id):max(datain.group_id)
	data_subset = datain(datain.group_id == ii,:);
	[interpol_subset] = descritize(data_subset, interval);
	rowA = rownum;
	rowB = rownum + height(interpol_subset) -1;
	temptable.group_id(rowA:rowB) = interpol_subset.group_id;
	temptable.counter(rowA:rowB) = interpol_subset.counter;
	temptable.x(rowA:rowB) = interpol_subset.x;
	temptable.y(rowA:rowB) = interpol_subset.y;
	rownum = rowB +1;
end
npts = sum(~isnan(temptable.x));
temptable = temptable(1:npts, :);


%%%%%%%%%%%%%%%%%%%%%%%%%%%
% make a plot for visual check
%%%%%%%%%%%%%%%%%%%%%%%%%%%

subplot(1,1,1)
for ii = 202 %min(datain.group_id):max(datain.group_id)
	mytable = datain(datain.group_id == ii,:);
	sortrows(mytable,'counter');
	plot(mytable.x, mytable.y, 'black', 'DisplayName', 'Line')
	hold on
end
box on
grid on
axis equal
ax = gca;
ax.XRuler.Exponent = 0;
ax.YRuler.Exponent = 0;
hold on
scatter(data_subset.x, data_subset.y, 'DisplayName', 'Original Spacing')
scatter(temptable.x, temptable.y, 14, 'filled', 'magenta', 'DisplayName', 'Resampled Spacing')
legend


%%%%%%%%%%%%%%%%%%%%%%%%%%%
% FUNCTION
%%%%%%%%%%%%%%%%%%%%%%%%%%%

function [table_interpol] = descritize(datatable, stepsize)
	% check inputs
	check = istable(datatable);
	if check ~= 1
		error('--- data input to descritize function must be a table ---');
	end
	check = length(unique(datatable.group_id));
	if check ~= 1
		error('--- data input to descritize function must have only one group_id ---');
	end
	% pre-allocate empty table
	cols = datatable.Properties.VariableNames;
	temp = array2table(NaN(50*1000,length(cols)));
	temp.Properties.VariableNames = cols;
	rowcounter = 1;
	
	% descritize the line
	sortrows(datatable,'counter');
	for jj = 1:max(datatable.counter)-1
		ptA = [datatable.x(jj), datatable.y(jj)];
		ptB = [datatable.x(jj+1), datatable.y(jj+1)];
		dist = sqrt((ptB(1) - ptA(1))^2 + (ptB(2) - ptA(2))^2);
		if dist > stepsize
			npts_new = ceil(dist/stepsize)+1;
			x_new = linspace(ptA(1), ptB(1), npts_new);
			y_new = linspace(ptA(2), ptB(2), npts_new);
		else
			npts_new = 2;
			x_new = [ptA(1), ptB(1)];
			y_new = [ptA(2), ptB(2)];
		end
		x_new = x_new';
		y_new = y_new';
		rowA = rowcounter;
		rowB = rowcounter + length(x_new) -1;
		temp.x(rowA:rowB) = x_new;
		temp.y(rowA:rowB) = y_new;
		%rowcounter = rowB +1;
		rowcounter = rowB; % do this to overwrite previous end, which is the next start
	end
	npts = sum(~isnan(temp.x));
	groupid = ones(npts, 1) .* unique(datatable.group_id);
	counterid = 1:1:npts;
	counterid = counterid';
	temp = temp(1:npts, :);
	temp.group_id = groupid;
	temp.counter = counterid;
	table_interpol = temp;
end