在matlab中用m_map画图画出来的图网格密集

问题遇到的现象和发生背景

问题相关代码，请勿粘贴截图

clear;
clc;
file = 'ETOPO1_Ice_g_gmt4.grd';
ncdisp(file);
lon = ncread(file,'x');
lat = ncread(file,'y');
depth = ncread(file,'z');
a = 123.324722+180;
b = 18.060278+90;
a1 = a/(1/60);
b1 = b/(1/60);

figure(1);
%变量输入绘图
varname = 'z';
boun = [120 125 10 20];
source = strcat(file);
lons = find(lon >= boun(1) & lon<=boun(2));
lats = find(lat >= boun(3) & lat<=boun(4));
lonn = length(lons);
latn = length(lats);
start = [lons(1),lats(1)];
count = [lonn,latn];
stride = [1,1];
dep = ncread(source,varname,start,count,stride);
%列优先
dep = imrotate(dep(:,:), 90);
%墨卡托投影
m_proj('Mercator','lat',[boun(3) boun(4)],'lon',[boun(1) boun(2)]);
%生成网格
lat1 = linspace(boun(3),boun(4),latn);
lon1 = linspace(boun(1),boun(2),lonn);
[m,n] =meshgrid(lon1',lat1');
n = flipud(n);
hold on
%绘制图形
dep(dep>0)=nan;
m_pcolor(m,n,dep)
m_coast('patch',[.7,.7,.7],'edgecolor','r');
m_grid('box','on')
title('mmap','fontsize',15)
hold on
colorbar();
hold on
[X,Y] = m_ll2xy(123,18);
line(X,Y,'marker','.','color','r','markersize',16);
text(X,Y,'M1','vertical','top')

运行结果及报错内容

我的解答思路和尝试过的方法

放大后得到的图片就比较清晰

我想要达到的结果