Python脚本驱动地理处理：自动化与逻辑关键

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本资源是一份名为《Geoprocessing Scripts with Python》的教程，它是学习ArcGIS Python编程的重要参考资料。该文档聚焦于在ArcGIS环境中利用Python进行地理空间数据处理和管理。主要内容涉及以下几个关键点： 1. **脚本与Python集成**： ArcGIS模型构建或数据管理过程中，往往需要执行一系列步骤来完成任务，如投影、裁剪到研究区域或进行数据融合等。为了高效处理大量工作并适应不同条件下的需求，脚本编程变得至关重要。通过编写Python脚本，可以自动化繁琐的操作，并运用逻辑驱动整个过程。自动化（Automation）和逻辑（Logic）是这里的核心概念。 2. **脚本编程的目的**：主要目标是将用户的高阶决策（比如针对不同类型的数据采取不同的处理方法）与低阶的自动化操作相结合，以提升工作效率。例如，在地理处理脚本中，可能会编写逻辑来区分栅格数据和矢量数据的处理方式，或者根据特定条件执行特定操作。 3. **交互与自动化**：在日常计算机交互中，我们通常通过电子邮件、文档编辑或地图设计等活动与计算机互动；而在地理空间数据分析时，我们需要转向自动化，利用逻辑指导数据处理流程，以便更有效地进行大规模数据处理。 4. **脚本中的决策与逻辑**：文档强调了在脚本编程中运用决策的重要性，这包括基于数据类型、属性或其他条件对数据进行差异化处理。通过Python代码，我们可以实现灵活且精确的控制，使系统能根据需要动态地调整处理方式。总结来说，《Geoprocessing Scripts with Python》是一份深入探讨如何在ArcGIS环境中使用Python语言编写地理空间处理脚本的指南，涵盖了自动化工作流和逻辑设计的核心技巧，对于提高GIS数据分析和管理效率具有显著价值。无论是初学者还是进阶用户，都可以从中学习到如何结合Python和ArcGIS进行高效地理信息处理。

1.6 Creating functions with def

Somewhat similar to a module, but much simpler, is a function you define in your code to be used

simply by using the function name later in your code, and providing any parameters you want to use.

The function can thus be used as a variable in your program. This is easier to understand with an

example. The following code defines a radians function that simply converts degrees to radians, and

also defines a degrees function that converts radians to degrees. There are similarly named built-in

functions in Excel.

 From here on out, we’ll want to write scripts and save them into your “mypy” folder. For each

script use Ctrl-n or File New to create a new script. Maximize your PythonWin screen so you can

see both the script and the interactive window (where outputs go). Then after composing it, save it

to your “mypy” folder, and run the script with

the run button. In some cases, I’ll suggest

names for your scripts; otherwise make one up.

 Try the following code in a script saved into

your d:/prog/py folder as defRadians.py.

import math

def radians(angdeg):

return angdeg * math.pi / 180

def degrees(angrad):

return angrad * 180 / math.pi

print math.sin(radians(45))

print degrees(math.acos(0.5))

 Note the use of the return keyword to specify what value to return to the function when you

use it. Thus the radians function returns the result of the expression angdeg * math.pi / 180

whenever the radians( ) function is evoked, with the value in parentheses being processed as

angdeg in the function definition. Note the colon and indentation.

 There are many other uses for function definitions, some you'll see in the later examples. In

this code, we provided constants (45 and 0.5), but can you see how you could use this kind of

capability in a program where your data might be read from files, etc.

 Trigonometric functions are commonly used when working with field survey measurements,

such as with a compass, and since compasses all read in degrees instead of radians the above

conversions are commonly needed.

? Challenge: How would you change the program code so that you could have the print

statements read:

print sin(radians(45))

print degrees(acos(0.5))

? Challenge: Though this would be unconventional, you could also set up your code to always

work in degrees. How would you write your code so that you could write the statements as:

print sin(45)

print acos(0.5)

Optional: try the same things with a scientific

calculator or use the built-in Windows

Calculator program (in Accessories) with the

View set to "Scientific" – you can set the

calculator to work in degrees or radians. On

the calculator, enter 45, make sure the Degrees

option is chosen, and press the sin button.

Write down the result to 5 or so decimal places.

Then enter 0.5 and press the Inv option and

then press the cos button. Compare its results to

the code here.

1.7 Control Flow Structures: If, While, For

An important feature of any scripting or programming language is the ability to execute a set of

statements as a group, but under controlled conditions.

Scenario: You would like to create a series of hillshade rasters to represent summer, winter and

equinox conditions. The hillshade tool requires inputs of sun angle (altitude) and azimuth. You could

look up values in a table, but why not have the computer derive these from what you know? We'll start

with a somewhat informed situation – we know what solar declination is, and what values for solar

declination are for four significant dates during the year:

Significant date Solar declination

June solstice (June 21) 23.44

Equinox (Mar. 21, Sept 21) 0

December solstice (Dec. 21) -23.44

The following is fairly simple code that derives sun angle and azimuth from solar declination (the

latitude where the sun’s rays are vertical at noon) and latitude. It starts with information to populate

two variables, lat (latitude for the area of study, negative if south of the equator) and decl (solar

declination), and from these derives sunangle and azimuth:

lat = 30

decl = 20

sunangle = 90 - lat + decl

azimuth = 180

if sunangle > 90:

sunangle = 180 - sunangle

azimuth = 0

print sunangle

For now, we’ve hard-coded the inputs of lat and decl. In this case sunangle would be assigned the

value 80 since 90 – 30 + 20 is evaluated as 80. The next line assigns 180 to the variable azimuth.

The last two lines assigns new values to sunangle and azimuth if sunangle ends up with value greater

than 90 after the first two lines are processed. Sun azimuth is either from the south (180) or from the

north (0) at solar noon.

There are three of these control flow operations:

if Only execute the statements if a particular condition is met.

while Keep executing (loop through) the set multiple times while a condition exists.

for Loop through the set for each value in a range of values.

These follow Python's clear syntax rules, similar to the def we just looked at:

 A colon is at the end of the initial statement.

 The block of statements to be processed is indented.

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