ValueError：提供两个参数时，数组的形状必须为（N，4）

Question

我一直在尝试使用pv-extractor软件包提取沿特定路径的速度。然后将像素坐标中的位置转换为世界坐标，并将频率转换为速度。这是我编写的代码：

###############################
## import packages
###############################
import matplotlib
matplotlib.use('Qt4Agg')

import matplotlib.pyplot as plt

from astropy.io import fits
from astropy import wcs
from astropy import coordinates
from astropy import units as u
import os
import aplpy
import numpy as np
from pvextractor import extract_pv_slice
from pvextractor.geometry import Path
from astropy import constants as const
import array as arr

###############################

fitsfile = fits.open('Image.fits', cache=True)
hdu = fitsfile[0]
data = hdu.data[0,:,:,:]
header = hdu.header
w = wcs.WCS(hdu.header)


###############################
## pv extract
###############################

from pvextractor import Path
list = [(1120.40,780.97), (881.37,741.97)]

path1 = Path(list, width=20 )
pv1 = extract_pv_slice(data, path1)

fig = plt.figure( )

F1 = aplpy.FITSFigure(pv1, figure=fig)

F1.show_colorscale(cmap='YlGnBu')

plt.show()


##################################
## define functions
##################################
restf = 8.6754290e10
f0 = header['CRVAL3']
f0 = np.float(f0)
df = header['CDELT3']
df = np.float(df)
ref = header['CRPIX3']
ref = np.float(ref)
chan_max = header['NAXIS3']
chan_max = np.float(chan_max)

channels = np.arange(0, chan_max) # generate array with number of channels


##################################
## velocity
##################################

f0=np.float(f0)
df=np.float(df)
freq = []
freq = f0 + ( ( channels - ref ) * df )



v = []
for i in xrange(0 , len(freq)):
    v.append(const.c.value * (1 - (freq[i] / restf)))




##################################
## distance 
##################################

pixcrd = np.array(list, np.float)

world = w.wcs_pix2world(pixcrd,1)

ListDist =[]
for i in renge (0, len(list)):
    Pixel_x_1 =world[i][0]
    Pixel_y_1 =world[i][1]
    print Pixel_x_1
    print Pixel_y_1
    try: 
       Pixel_x_2 =world[i+1][0]
       Pixel_y_2 =world[i+1][1]
    except IndexError:
       break

    dist_dc = abs(Pixel_y_2 - Pixel_y_1)
    dist_ra = abs(Pixel_x_2 - Pixel_x_1) * np.cos(())
    ListDist.append(dist_connect)
        print ListDist
dist_total = sum(ListDist)  #it should be in arcsec

print dist_total
###############################

它完美地提取了路径并显示了地图，我也可以提取速度，但是在最后一步中，我遇到了以下错误，但我不知道如何解决：

>>Traceback (most recent call last):

>> File "distance.py", line 43, in <module>

>>   world = w.wcs_pix2world(pixcrd,1)

>> File "/usr/lib64/python2.7/site-packages/astropy/wcs/wcs.py", line 1355, in wcs_pix2world
>>    'output', *args, **kwargs)
>>  File "/usr/lib64/python2.7/site-packages/astropy/wcs/wcs.py", line 1257, in _array_converter
 >>   return _return_single_array(xy, origin)
>>  File "/usr/lib64/python2.7/site-packages/astropy/wcs/wcs.py", line 1238, in _return_single_array
 >>   "of shape (N, {0})".format(self.naxis))

>>ValueError: When providing two arguments, the array must be of shape (N, 4)