In my last article, I discussed the steps to download NASA data from GES DISC. The data files downloaded are in the HDF5 format. HDF5 is a file format, a technology, that enables the management of very large data collections. Thus, it is quite popular for storing information. For getting NASA’s data, please check the below article first:

在上一篇文章中，我讨论了从GES DISC下载NASA数据的步骤。 下载的数据文件为HDF5格式。 HDF5是一种文件格式，一种技术，可以管理非常大的数据集。 因此，在存储信息方面非常流行。 要获取NASA的数据，请先查看以下文章：

Whenever I work with datasets, I’m most comfortable with CSV files. Thus, once I got the HDF5 files, I decided to look for ways to change them to CSV files. I found the package h5py in Python, which enables the reading in of HDF5 files. Thus, this article articulates the steps to use h5py and convert HDF5 to CSV. You can follow along by referring to the complete notebook at the link below.

每当我使用数据集时，我对CSV文件都很满意。 因此，一旦获得HDF5文件，我便决定寻找将其更改为CSV文件的方法。 我在Python中找到了h5py软件包，该软件包可以读取HDF5文件。 因此，本文阐述了使用h5py并将HDF5转换为CSV的步骤。 您可以通过以下链接参考完整的笔记本。

导入库 (Import libraries)

For this work, we’ll require two libraries. The first library is h5py which has the option to read and work with HDF5 files (documentation). The second package we need is numpy to work with arrays. Finally, we will import pandas so we can create a dataframe and later save it as a CSV file.

对于这项工作，我们将需要两个库。 第一个库是h5py ，它具有读取和使用HDF5文件( 文档 )的选项。 我们需要的第二个包是使用numpy来处理数组。 最后，我们将导入pandas以便我们可以创建一个数据框，然后将其另存为CSV文件。

import h5py
import numpy as np
import pandas as pd

加载数据集 (Load dataset)

The next step is to load in the HDF5 file. Note that for this example, I’m working with GPM data collected from GES DISC for January, 2020 for the whole world. It’s located inside the data folder in the GitHub repo (downloaded from GES DISC website).

下一步是加载HDF5文件。 请注意，在此示例中，我正在使用从GES DISC收集的2020年1月全世界的GPM数据。 它位于GitHub存储库中的data文件夹内(可从GES DISC网站下载)。

I’ll use the File method from the h5py library to read the HDF5 file and save it to the variable called dataset. I specify the name of the file along with the full path as the first argument and set the second argument as r indicating that I’m working with this file in the read only mode.

我将使用h5py库中的File方法读取HDF5文件并将其保存到名为dataset的变量中。 我将文件名以及完整路径指定为第一个参数，并将第二个参数设置为r指示我正在以只读模式使用此文件。

dataset = h5py.File('data/gpm_jan_2020.HDF5', 'r')

探索数据集 (Explore dataset)

Now that the file is all loaded in, we can get started with exploring the dataset. The elements of this file are similar to a Python dictionary. Thus, it comprises of key-value pairs. So, I started by looking at the various keys in this file.

现在文件已全部加载完毕，我们可以开始探索数据集了。 该文件的元素类似于Python字典。 因此，它包含键值对。 因此，我首先查看了此文件中的各种键。

dataset.keys()## Output
# <KeysViewHDF5 ['Grid']>

As we see in the result above, there is just one key called Grid. As we do not see any data, the data might be inside the value of this key; so that’s what I do, I read its value using dataset[’Grid’] into grid and look at its keys.

正如我们在上面的结果中看到的，只有一个名为Grid键。 由于我们看不到任何数据，因此该数据可能在此键的值之内； 这就是我的工作，我使用dataset['Grid']将其值读取到grid并查看其键。

grid = dataset['Grid']
grid.keys()## Output
# <KeysViewHDF5 ['nv', 'lonv', 'latv', 'time', 'lon', 'lat', 'time_bnds', 'lon_bnds', 'lat_bnds',
# 'precipitation', 'randomError', 'gaugeRelativeWeighting', 'probabilityLiquidPrecipitation',
# 'precipitationQualityIndex']>

Finally, we see the data. We have several features that we can use. All of these represent keys and they will have their corresponding values, which is what we’re looking for. For this work, I’m only interested in latitude, longitude and the precipitation at that latitude-longitude combination.

最后，我们看到了数据。 我们有几个可以使用的功能。 所有这些都代表键，并且它们将具有它们对应的值，这就是我们想要的。 对于这项工作，我只对纬度，经度和该纬度-经度组合处的降水感兴趣。

Let’s take a look at each of these features one by one.

让我们一一看一下这些功能。

经度 (Longitude)

I print the longitude key and its attributes. We find that there are 3600 values for longitude. In the attributes, the units and standard_name are the two things I would like to use.

我打印经度键及其属性。 我们发现经度有3600个值。 在属性中， units和standard_name是我要使用的两件事。

print("Longitude data: {}".format(grid['lon']))
print("Longitude data attributes: {}".format(list(grid['lon'].attrs)))## Output
# Longitude data: <HDF5 dataset "lon": shape (3600,), type "<f4">
# Longitude data attributes: ['DimensionNames', 'Units', 'units',
# 'standard_name', 'LongName', 'bounds', 'axis', 'CLASS', 'REFERENCE_LIST']print("Name: {}".format(grid['lon'].attrs['standard_name'].decode()))
print("Unit: {}".format(grid['lon'].attrs['units'].decode()))## Output
# Name: longitude
# Unit: degrees_east

纬度 (Latitude)

Similar to longitude, I check the key and its attributes. There are 1800 latitude values and units and standard_name are useful to me.

与经度类似，我检查键及其属性。 有1800个纬度值， units和standard_name对我有用。

print("Latitude data: {}".format(grid['lat']))
print("Latitude data attributes: {}".format(list(grid['lat'].attrs)))## Output
# Latitude data: <HDF5 dataset "lat": shape (1800,), type "<f4">
# Latitude data attributes: ['DimensionNames', 'Units', 'units',
# 'standard_name', 'LongName', 'bounds', 'axis', 'CLASS', 'REFERENCE_LIST']print("Name: {}".format(grid['lat'].attrs['standard_name'].decode()))
print("Unit: {}".format(grid['lat'].attrs['units'].decode()))## Output
# Name: latitude
# Unit: degrees_north

沉淀 (Precipitation)

I print the key and attributes for precipitation.

我打印沉淀的键和属性。

print("Precipitation data: {}".format(grid['precipitation']))
print("Precipitation data attributes: {}".format(list(grid['precipitation'].attrs)))## Output
# Precipitation data: <HDF5 dataset "precipitation": shape (1, 3600, 1800), type "<f4">
# Precipitation data attributes: ['DimensionNames', 'Units', 'units', 'coordinates',
# '_FillValue', 'CodeMissingValue', 'DIMENSION_LIST']print("Unit: {}".format(grid['precipitation'].attrs['units'].decode()))## Output
# Unit: mm/hr

The precipitation data is a 3-dimensional array with the precipitation values stored as a 2-d matrix. It has the shape (3600, 1800) meaning it has precipitation values across all combinations of longitude and latitude. Further, the units of precipitation are found in the units attribute as mm/hr.

降水量数据是一个3维数组，其中降水量值存储为2-d矩阵。 它的形状为(3600，1800)，表示在所有经度和纬度组合中都具有降水值。 此外，在units属性中以mm/hr为units找到降水的units 。

创建数据框 (Create dataframe)

As we now know all the things we want to capture in the final CSV file, we’ll directly proceed with capturing that.

现在我们知道了要在最终CSV文件中捕获的所有内容，我们将直接进行捕获。

Our dataset will have 6,480,000 rows (3600x1800). Each of these rows has a unique combination of longitude and latitude. Thus, for each longitude, we have a latitude and the corresponding precipitation value.

我们的数据集将有6,480,000行(3600x1800)。 这些行中的每行都有经度和纬度的唯一组合。 因此，对于每个经度，我们都有一个纬度和相应的降水值。

To create the list of longitude values for all precipitation values, we need to repeat each value of the longitude list 1800 times. This is saved as longitude_values with a total length of 6,480,000 values (3600 longitude values, each repreated 1800 times) using np.repeat().

要为所有降水值创建经度值列表，我们需要将经度列表的每个值重复1800次。 使用np.repeat()将其保存为longitude_values值，总长度为6,480,000个值(3600个经度值，每个值被np.repeat() 1800次np.repeat() 。

For each longitude value, we need all latitude values. So, to create the final latitude_values list, we multiply the complete list by 3600, creating a 6,480,000 long list with latitude list repeated over and over again 3600 times.

对于每个经度值，我们需要所有纬度值。 因此，要创建最终的latitude_values列表，我们将整个列表乘以3600，创建一个6,480,000长的列表，并将纬度列表重复3600次。

Finally, to convert the matrix of precipitation values, I used the flatten() function to convert it into a long list.

最后，为了转换降水值矩阵，我使用了flatten()函数将其转换为一个长列表。

longitude_values = np.repeat(list(grid['lon']), 1800)
latitude_values = list(grid['lat'])*3600
precipitation_values = np.array(list(grid['precipitation'])).flatten()dataset = pd.DataFrame({"lon": longitude_values, "lat": latitude_values, "precipitation": precipitation_values})
dataset.columns = [grid['lon'].attrs['standard_name'].decode() + " (" + grid['lon'].attrs['units'].decode() + ")",grid['lat'].attrs['standard_name'].decode() + " (" + grid['lat'].attrs['units'].decode() + ")","Precipitation (" + grid['precipitation'].attrs['units'].decode() + ")",]
dataset.head()

I then saved these lists as columns of the dataset dataframe with labels lon, lat and precipitation. I rename the columns with the labels and the units we extracted above. Note that the strings are saved with a b due to byte encoding, so I append these attributes with decode() to get the string values.

然后，我将这些列表另存为dataset数据框的列，标签为lon ， lat和precipitation 。 我使用上面提取的标签和单位对列进行重命名。 请注意，由于字节编码的原因，字符串使用b进行保存，因此我将这些属性附加到decode()以获取字符串值。

Some precipitation values are not captured or are missing and are denoted by -9999.900391 in the dataset, so I mask them with a value 0.

一些降水量值未捕获或丢失，在数据集中用-9999.900391表示，因此我将其掩码为0 。

dataset['Precipitation (mm/hr)'] = dataset['Precipitation (mm/hr)'].mask(dataset['Precipitation (mm/hr)'] == -9999.900391, 0)

As the last step, I saved the dataframe as a CSV file precipitation_jan_2020.csv inside the data folder.

作为最后一步，我保存的数据帧为CSV文件precipitation_jan_2020.csv 数据文件夹内。

dataset.to_csv("data/precipitation_jan_2020.csv", index = False)

And that’s it! We have our CSV file.

就是这样！ 我们有CSV文件。

结论 (Conclusion)

In this article, I described the steps to load a HDF5 file in Python, read its elements and finally create a dataframe that can be saved as a CSV file. The same steps can be replicated for any other HDF5 file as well.

在本文中，我描述了在Python中加载HDF5文件，读取其元素并最终创建可保存为CSV文件的数据框的步骤。 相同的步骤也可以复制到任何其他HDF5文件。

Hope you liked this article. If you have any questions, suggestions or ideas, please mention them in the comments.

希望您喜欢这篇文章。 如果您有任何问题，建议或想法，请在评论中提及。