使用ArcGIS编写地理处理脚本

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"编写地理处理脚本" 在GIS领域，地理处理是分析、操作和管理地理数据的核心过程。ArcGIS提供了一种强大的工具，允许用户通过编写脚本来执行复杂的地理处理任务。"Writing_Geoprocessing_Scripts" 主题旨在帮助用户掌握如何利用ArcGIS平台编写自定义的地理处理脚本，提高工作效率并实现自动化处理。地理处理脚本通常使用Python语言编写，因为ArcGIS有一个内置的Python解释器，使得与GIS功能交互变得简单。Python的易读性和灵活性使其成为地理处理任务的理想选择。通过编写脚本，用户可以创建自定义工具，自动化数据转换、空间分析、模型构建等操作，而无需手动进行每个步骤。在ArcGIS中，地理处理脚本通常涉及到以下几个关键知识点： 1. **ArcPy库**：这是Esri提供的Python模块，包含了用于访问ArcGIS地理处理服务的函数和类。通过ArcPy，用户可以调用地理处理工具，如栅格分析、矢量操作、投影转换等。 2. **数据访问**：使用ArcPy，你可以读取、写入和修改各种GIS数据格式，如Shapefiles、Geodatabases、栅格数据集等。这包括对属性表的操作，如添加、修改或删除字段，以及查询和筛选数据。 3. **工作流管理**：编写脚本时，你需要组织一系列的地理处理步骤，形成一个工作流。这可能涉及使用`arcpy.GP`模块的工具对象，或者创建更复杂的`ModelBuilder`模型并将其导出为Python脚本。 4. **参数和输入**：地理处理脚本通常需要接收用户输入，如文件路径、参数值等。ArcPy提供了处理这些输入的方法，使脚本更具交互性。 5. **错误处理和日志记录**：为了确保脚本的健壮性，你需要考虑错误处理，捕获可能出现的问题，并提供适当的反馈。同时，日志记录有助于追踪脚本的运行情况，方便调试和优化。 6. **脚本工具**：在ArcGIS中，你可以将Python脚本封装为脚本工具，这样它们就可以在ArcGIS Desktop或ArcGIS Pro的工具箱中像其他预定义工具一样使用。这增加了脚本的可重用性和用户友好性。 7. **地理处理服务**：如果你需要在服务器环境中执行脚本，例如通过ArcGIS Server发布地理处理服务，你需要了解如何配置和部署这些服务。通过学习和实践"Writing_Geoprocessing_Scripts"，GIS专业人员能够扩展ArcGIS的功能，解决特定的业务问题，实现数据管理和分析的自动化，从而提高工作效率和数据质量。记住，理解和熟练运用这些知识需要时间和实践，但是一旦掌握，你将能够更有效地处理大量地理数据和复杂的分析任务。

Chapter 2 • Getting started • 15

a. Strings are indexed, with the first character being 0. A set of characters may be

accessed by specifying a range. Specifying “a[0:3]” would return “Bud”.

9. Type “print len(b)” and press Enter. This returns the length, or number of

characters, in variable b.

There are many tools for working with strings in Python and many techniques for

extracting, repeating, and formatting them. Refer to your Python reference for

more information on working with strings.

10. Click the Interactive window button

on the Standard toolbar to close it.

Importing Modules

By definition, Python scripts are modules,

which is Python’s highest level of code organi-

zation. Modules serve several purposes; the

most obvious is the ability to save code to a

reusable form, a script. Scripts may avoid

duplication of code by using programs con-

tained in other modules. Using the import

statement, a program may import any number

of functions, variables, or both contained in

another module. A module creates a natural

grouping and naming structure, which elimi-

nates ambiguity when using commonly named

programs.

Each time you create a script, you will import

one or more modules. All geoprocessing scripts

use the win32com module, as it provides the

programs needed to support the communica-

tion between Python and the COM IDispatch interface, which is used by the

geoprocessor. This is covered in more detail in Chapter 3. Other modules that are

required to work with math, files, and the operating system may also be im-

ported, depending on the requirements of your script.

AN INTRODUCTION TO PYTHON AND PYTHONWIN

The string module provides many

tools for string manipulation, such as

searching, conversion, and formatting. You

can access these tools by importing the

string module within your script. Many of

these tools are also found as methods of

a string object, so the string module is

not always needed.

16 • Writing Geoprocessing Scripts With ArcGIS

Creating a new script module

1. In PythonWin click the File menu and click the New option. Accept the

default option of Python Script and click OK.

The Script1 window will open. Script1 is the default name of your script.

2. Click the maximize window button on the Script1 window.

3. Click the File menu and click Save As. Name the script “multi_clip” and save it

in a folder of your choice.

Modules should follow the same naming rules as variables. They must start with a

letter and can be followed by any number of letters, digits, or underscores. Mod-

ule names become variable names when they are imported, so to avoid syntax

errors, they should not use reserved words such as while or if. Scripts should

always have a .py extension, as this is required when importing a script module. It

also defines Python as the executing application for your script.

4. At the top of the code window add the following lines:

#Import standard library modules

import win32com.client, sys, os

This imports the system, operating system, and Windows 32 modules to the

script. The win32com module has already been discussed, but the other two are

new. The sys module refers to the Python system and will be used to access

user-specified inputs. The os module provides easy access to the most fundamen-

tal tools of the operating system. Some of the os module’s filename manipula-

tion tools are used in this script. Refer to your Python reference material for more

information about the tools in these modules.

5. Add the following code to declare the geoprocessor object:

#Create the Geoprocessor object

GP = win32com.client.Dispatch("esriGeoprocessing.GpDispatch.1")

This script will have the following arguments so it can be used in a generic fash-

ion:

• An input workspace defining the set of feature classes to process

• A feature class to be used by the Clip tool as the area to be clipped from an

input feature class

• An output workspace where the results of the Clip tool will be written

• A cluster tolerance that will be used by the Clip tool

Refer to the Clip tool in the Analysis toolbox for detailed information on how

Clip works.

6. Add the following code to your script to define and set variables based on the

user-defined values passed to the script at execution:

#Set the input workspace

GP.workspace = sys.argv[1]

#Set the clip featureclass

clipFeatures = sys.argv[2]

CREATING A SCRIPT

See Chapter 3 for a complete de-

scription of the geoprocessor object

and how to work with its properties

and methods.

When naming variables, be mindful

that Python is case senitive, so “GP”

is not the same as “gp”. The names of

the geoprocessor’s methods and

properties (including tool and envi-

ronment setting names) are NOT

case sensitive.

Python automatically removes an

object from memory when it is no

longer referenced in a program.

When a script completes, all memory

allocated for objects is released and

any open files are closed. The del

statement can be used to delete an

object so its memory is deallocated

during a script’s execution.

Chapter 2 • Getting started • 17

CREATING A SCRIPT

#Set the output workspace

outWorkspace = sys.argv[3]

#Set the cluster tolerance

clusterTolerance = sys.argv[4]

The passed-in arguments are attributes of the sys module and can be sliced to

extract the argument you want. The first value of argv, argv[0], is the name of

the script. You can apply the same slicing tools used on strings to the argv at-

tribute, so sys.argv[1:] will return the entire list of argument values.

7. Add the following error handling statement and geoprocessor call to the script

window:

try:

#Get a list of the featureclasses in the input folder

fcs = GP.ListFeatureClasses()

Python enforces indentation of code after certain statements as a construct of

the language. The try statement defines the beginning of a block of code that

will be handled by its associated exception handler, or except statement. All

code within this block must be indented. Python uses try/except blocks to

handle unexpected errors during execution. Exception handlers define what the

program should do when an exception is raised by the system or by the script

itself. In this case you are only concerned about an error occurring with the

geoprocessor, so a try block is started just before you start to use the object. It is

good practice to use exception handling in any script using the geoprocessor so its

error messages can be propagated back to the user. This also allows the script to

exit gracefully and return informative messages instead of simply causing a system

error.

The geoprocessor’s ListFeatureClasses method returns an enumeration of feature

class names in the current workspace. The workspace defines the location of your

data and where all new data will be created unless a full path is specified. The

workspace has already been set to the first argument’s value. Chapter 4 discusses

how to work with enumerations in detail.

8. Add the following code to get the first value of the enumeration:

#Loop through the list of feature classes

fcs.Reset()

fc = fcs.Next()

An enumeration is a list that has an undefined number of items. It should always

be reset before the first item is extracted so you can be sure you get the first item

in the list. You want to get the first item before you start looping through its

contents as you will use it as the conditional value for continuing a loop.

9. Add the following code:

while fc:

#Validate the new feature class name for the output workspace.

outFeatureClass = outWorkspace + "/" + GP.ValidateTableName(fc,

outWorkspace)

#Clip each feature class in the list with the clip feature class.

Many geoprocessing scripts are not

generic in nature and have no argu-

ments. They use defined dataset

names and parameter values and may

not require the sys module. Import

modules only as needed to avoid

unnecessary memory use.

Statements that end with a colon indi-

cate the beginning of indented code.

Python does not use braces, brackets, or

semicolons to indicate the beginning or

end of a block of code. Instead Python

uses the indentation of the block to

define its boundaries. This results in code

that is easy to read and write.

18 • Writing Geoprocessing Scripts with ArcGIS

CREATING A SCRIPT

#Do not clip the clipFeatures, it may be in the same workspace.

if str(fc) != str(os.path.split(clipFeatures)[1]):

GP.Clip(fc, clipFeatures, outFeatureClass,

clusterTolerance)

fc = fcs.Next()

When there are no more names in the enumeration, a null or empty string will be

returned. Python will evaluate this as false, which will cause the while loop to

exit. The next value from the enumeration must be retrieved before the end of

the indented block so it is evaluated correctly at the start of the loop. The

ValidateTableName method is used to ensure the output name is valid for the

output workspace. Certain characters, such as periods or dashes, are not allowed

in geodatabases, so this method will return a name with valid characters in place

of invalid ones. It will also return a unique name so no existing data is overwrit-

ten. Chapter 7 fully explains how to use this method and how to work with the

geodatabase.

The os module’s path object is used to manipu-

late the clip feature classes path, so only the

feature class name is evaluated in an expression,

not the entire path. The Clip tool is accessed as a

method of the geoproccessor, using the various

string variables as parameter values.

10. Add the following lines to complete the

script:

except:

GP.AddMessage(GP.GetMessages(2))

print GP.GetMessages(2)

The except statement is required by the earlier

try statement, otherwise a syntax error will

occur. If an error occurs during execution, the

code within the except block will be executed.

In this case, any message with a severity value of

2, indicating an error, will be added to the

geoprocessor in case the script is used as the

source of a tool. All error messages are also

printed to the standard output in case the script

is run outside a tool.

11. Save the script by clicking the Save button

on the Standard toolbar.

12. Add the following comments to the top of

your script:

##Script Name: Clip Multiple Featureclasses

##Description: Clips one or more shapefiles

##from a folder and places the clipped

##feature classes into a Geodatabase.

##Created By: Insert name here.

##Date: 11/04/2003

Scripts should be well commented. Each script

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