Try one of these two things, depending on whether x_pts corresponds to the rows or columns of wmax:
% if x_pts corresponds to the rows of wmax and y_pts is the columns:
wmax(x_pts>-103 & x_pts<-100, y_pts>34.75 & y_pts<36.5);
% if x_pts corresponds to the columns of wmax and y_pts is the rows:
wmax(y_pts>34.75 & y_pts<36.5, x_pts>-103 & x_pts<-100);
Example:
x_pts = 1:10;
y_pts = 102:109;
% x_pts goes with rows, y_pts goes with columns, in this case
wmax = randn(numel(x_pts),numel(y_pts))
wmax(x_pts>3 & x_pts<7, y_pts>105 & y_pts<108)
