Reverse Array in a dataframe-CodePudding

Hi I am trying to extract data from a netCDF file, but the data is upside down. How can I reverse the database:

The data I want to extract is the height data from the (netcdf) at the points I have in the CSV file. my Data:

import numpy as np
from netCDF4 import Dataset
import matplotlib.pyplot as plt
import pandas as pd
from mpl_toolkits.basemap import Basemap
from matplotlib.patches import Path, PathPatch

csv_data = np.loadtxt('CSV with target coordinates',skiprows=1,delimiter=',')
num_el = csv_data[:,0]
lat = csv_data[:,1]
lon = csv_data[:,2]
value = csv_data[:,3]

data = Dataset("elevation Data",'r')
lon_range = data.variables['x_range'][:]
lat_range = data.variables['y_range'][:]
topo_range = data.variables['z_range'][:]
spacing = data.variables['spacing'][:]
dimension = data.variables['dimension'][:]
z = data.variables['z'][:]
lon_num =  dimension[0]
lat_num =  dimension[1]

etopo_lon = np.linspace(lon_range[0],lon_range[1],dimension[0])
etopo_lat = np.linspace(lat_range[0],lat_range[1],dimension[1])
topo = np.reshape(z, (lat_num, lon_num))

height = np.empty_like(num_el)
desired_lat_idx = np.empty_like(num_el)
desired_lon_idx = np.empty_like(num_el)
for i in range(len(num_el)): 
    tmp_lat = np.abs(etopo_lat - lat[i]).argmin()
    tmp_lon = np.abs(etopo_lon - lon[i]).argmin()
    desired_lat_idx[i] = tmp_lat
    desired_lon_idx[i] = tmp_lon
    height[i] = topo[tmp_lat,tmp_lon]
height[height<-10]=0 

print(len(desired_lat_idx))
print(len(desired_lon_idx))
print(len(height))


dfl= pd.DataFrame({
    'Latitude' : lat.reshape(-1),
    'Longitude': lon.reshape(-1),
    'Altitude': height.reshape(-1)
});
print(dfl)

# but the Lat should not be changed here (the dfl must be correct)
df =dfl
lat=np.array(df['Latitude'])
lon=np.array(df['Longitude'])
val=np.array(df['Altitude'])

m = basemap.Basemap(projection='robin', lon_0=0, lat_0=0, resolution='l',area_thresh=1000)
m.drawcoastlines(color = 'black')
x,y = m(lon,lat)
colormesh= m.contourf(x,y,val,100, tri=True,  cmap = 'terrain')
plt.colorbar(location='bottom',pad=0.04,fraction=0.06)
plt.show()

I have already tried:

lat = csv_data[:,1]
lat= lat*(-1)

But this didn´t work

CodePudding user response：

It's a plotting artifact().

Just do:

colormesh= m.contourf(x,y[::-1],val,100, tri=True,  cmap = 'terrain')

y[::-1] will reverse the order of the y latitude elements (as opposed to the land-mass outlines; and while keeping the x longitude coordinates the same) and hence flip them.

I've often had this problem with plotting numpy image data in the past.

Your raw CSV data are unlikely to be flipped themselves (why would they be?). You should try sanity-checking them [I am not a domain expert I'm afraid]! Overlaying an actual coordinate grid can help with this.

Another way to do it is given here: Reverse Y-Axis in PyPlot

You could also therefore just do

ax = plt.gca()
ax.invert_yaxis()