系列简介
本系列主要记录笔者在学习《python地理空间分析指南》一书中遇到的问题及收获。《python地理空间分析指南》电子书及其附带源码分享在：妙妙屋，提取码为：bzlx，以供有兴趣的朋友参考学习。
内容简介
本文主要记录《python地理空间分析指南》一书第一章的示例及本人的一点注记。
代码部分
"""SimpleGIS.py - A Simple GIS"""
import turtle as t
# DATA MODEL
# All layers will have a name, 1+ points, and population count
NAME = 0
POINTS = 1
POP = 2
# Create the state layer
state = ["COLORADO", [[-109, 37], [-109, 41], [-102, 41], [-102, 37]], 5187582]
# Cities layer list
# city = [name, [point], population]
cities = []
# Add Denver
cities.append(["DENVER", [-104.98, 39.74], 634265])
# Add Boulder
cities.append(["BOULDER", [-105.27, 40.02], 98889])
# Add Durango
cities.append(["DURANGO", [-107.88, 37.28], 17069])
# MAP GRAPHICS RENDERING
map_width = 800
map_height = 500
# State Bounding Box
# Use Python min/max function to get bounding box
minx = 180
maxx = -180
miny = 90
maxy = -90
for x, y in state[POINTS]:
if x < minx:
minx = x
elif x > maxx:
maxx = x
if y < miny:
miny = y
elif y > maxy:
maxy = y
# Get earth distance on each axis
dist_x = maxx - minx
dist_y = maxy - miny
# Scaling ratio each axis
# to map points from world to screen
x_ratio = map_width / dist_x
y_ratio = map_height / dist_y
def convert(point):
"""Convert lat/lon to screen coordinates"""
lon = point[0]
lat = point[1]
x = map_width - ((maxx - lon) * x_ratio)
# x = lon * x_ratio - minx * x_ratio
y = map_height - ((maxy - lat) * y_ratio)
# y = lat * y_ratio - miny * y_ratio
# 相当于以左下角点为参考
# Python turtle graphics start in the middle of the screen
# so we must offset the points so they are centered
x = x - (map_width/2)
# x= (lon - (maxx - minx)/2)*x_ratio
y = y - (map_height/2)
# y= (lat - (maxy - miny)/2)*x_ratio
# 结果为以最大值与最小值的中点为参考
return [x, y]
# Add a title to the window
wn = t.Screen()
wn.title("Simple GIS")
# Draw the state
t.up()
first_pixel = None
for point in state[POINTS]:
pixel = convert(point)
if not first_pixel:
first_pixel = pixel
t.goto(pixel)
t.down()
# Go back to the first point
t.goto(first_pixel)
# Label the state
t.up()
t.goto([0, 0])
t.write(state[NAME], align="center", font=("Arial", 16, "bold"))
# Draw the cities
for city in cities:
pixel = convert(city[POINTS])
t.up()
t.goto(pixel)
# Place a point for the city
t.dot(10)
# Label the city
t.write(city[NAME] + ", Pop.: " + str(city[POP]), align="left")
t.up()
# Perform an attribute query
# Question: Which city has the largest population?
# Write the result but make sure it's under the map
biggest_city = max(cities, key=lambda city: city[POP])
t.goto(0, -1*((map_height/2)+20))
t.write("The highest-populated city is: " + biggest_city[NAME])
# Perform a spatial query
# Question: Which is the western most city?
# Write the result but make sure it's under the other question
western_city = min(cities, key=lambda city: city[POINTS])
t.goto(0, -1*((map_height/2)+40))
t.write("The western-most city is: " + western_city[NAME])
# Hide our map pen
t.pen(shown=False)
t.done()
注记
其中个人认为以下内容需要注意：
def convert(point):
"""Convert lat/lon to screen coordinates"""
lon = point[0]
lat = point[1]
x = map_width - ((maxx - lon) * x_ratio)
# x = lon * x_ratio - minx * x_ratio
y = map_height - ((maxy - lat) * y_ratio)
# y = lat * y_ratio - miny * y_ratio
# 相当于以左下角点为参考
# Python turtle graphics start in the middle of the screen
# so we must offset the points so they are centered
x = x - (map_width/2)
# x= (lon - (maxx - minx)/2)*x_ratio
y = y - (map_height/2)
# y= (lat - (maxy - miny)/2)*x_ratio
# 结果为以最大值与最小值的中点为参考
return [x, y]
此函数实现将实际坐标转化为图上坐标，包括比例变化和参考点变化。
x = map_width - ((maxx - lon) * x_ratio)
# x = lon * x_ratio - minx * x_ratio
y = map_height - ((maxy - lat) * y_ratio)
# y = lat * y_ratio - miny * y_ratio
# 相当于以左下角点为参考
上方代码中注释部分为原表达式变形后所得，可以看出此时参考原点为左下角点。
x = x - (map_width/2)
# x= (lon - *(maxx - minx)/2)*x_ratio
y = y - (map_height/2)
# y= (lat - *(maxy - miny)/2)*x_ratio
# 结果为以最大值与最小值的中点为参考
上方代码中注释部分为原表达式变形后所得，可以看出此时参考原点变为为最大值与最小值的中点。