you can create a grid with a higher resolution and draw a contour plot.

code :

```
import numpy as np
import matplotlib.pyplot as plt
X,Y = np.meshgrid(range(30),range(30))
Z = np.sin(X)+np.sin(Y)
resolution = 25
f = lambda x,y: Z[int(y),int(x) ]
g = np.vectorize(f)
x = np.linspace(0,Z.shape[1], Z.shape[1]*resolution)
y = np.linspace(0,Z.shape[0], Z.shape[0]*resolution)
X2, Y2= np.meshgrid(x[:-1],y[:-1])
Z2 = g(X2,Y2)
plt.pcolormesh(X,Y, Z)
plt.contour(X2,Y2,Z2, [1.5], colors='r', linewidths=[1])
plt.show()
```