【GDAL】python读取DEM计算坡度与坡向

      利用GDAL读入DEM与Landsat影像，由于DEM是WG84坐标系，Landsat是WGS84坐标系UTM投影，因此处理在实际应用中需要将DEM进行投影转换。

大概分为以下几个步骤：

1. 读取DEM，读取Landsat影像
2. 获取Landsat影像的投影信息，将其赋给DEM，并对DEM进行重采样
3. 计算dx和dy
4. 计算坡度和坡向
5. 输出坡度和坡向的影像

 

from osgeo import gdal
import os
import sys
import osr
import math
import numpy as np
from matplotlib import pyplot as plt
from gdalconst import *


# 给栅格最外圈加一圈
def assignBCs(elevGrid):
    ny, nx = elevGrid.shape
    Zbc = np.zeros((ny + 2, nx + 2))
    Zbc[1:-1, 1:-1] = elevGrid

    Zbc[0, 1:-1] = elevGrid[0, :]
    Zbc[-1, 1:-1] = elevGrid[-1, :]
    Zbc[1:-1, 0] = elevGrid[:, 0]
    Zbc[1:-1, -1] = elevGrid[:, -1]

    Zbc[0, 0] = elevGrid[0, 0]
    Zbc[0, -1] = elevGrid[0, -1]
    Zbc[-1, 0] = elevGrid[-1, 0]
    Zbc[-1, -1] = elevGrid[-1, 0]

    return Zbc


# 计算dx,dy
def calcFiniteSlopes(elevGrid, dx):
    Zbc = assignBCs(elevGrid)

    Sx = (Zbc[1:-1, :-2] - Zbc[1:-1, 2:]) / (2 * dx)  # WE方向
    Sy = (Zbc[2:, 1:-1] - Zbc[:-2, 1:-1]) / (2 * dx)  # NS方向

    return Sx, Sy


# 投影转换
def convertProjection(data, filename):
    landsatData = gdal.Open(filename, GA_ReadOnly)

    oldRef = osr.SpatialReference()
    oldRef.ImportFromWkt(data.GetProjectionRef())

    newRef = osr.SpatialReference()
    newRef.ImportFromWkt(landsatData.GetProjectionRef())

    transform = osr.CoordinateTransformation(oldRef, newRef)

    tVect = data.GetGeoTransform()
    nx, ny = data.RasterXSize, data.RasterYSize
    (ulx, uly, ulz) = transform.TransformPoint(tVect[0], tVect[3])
    (lrx, lry, lrz) = transform.TransformPoint(tVect[0] + tVect[1] * nx, tVect[3] + tVect[5] * ny)

    memDrv = gdal.GetDriverByName('MEM')

    dataOut = memDrv.Create('name', int((lrx - ulx) / dx), int((uly - lry) / dx), 1, gdal.GDT_Float32)

    newtVect = (ulx, dx, tVect[2], uly, tVect[4], -dx)

    dataOut.SetGeoTransform(newtVect)
    dataOut.SetProjection(newRef.ExportToWkt())

    # Perform the projection/resampling
    res = gdal.ReprojectImage(data, dataOut, oldRef.ExportToWkt(), newRef.ExportToWkt(), gdal.GRA_Cubic)

    return dataOut


if __name__ == '__main__':
    DEMFilename = 'E:/LandsatDEM/dem/DEM.tif'
    LandsatFilename = 'E:/LandsatDEM/clip/L7-B1.tif'
    slopeFilename = 'E:/LandsatDEM/result/slope_prj.tif'
    aspectFilename = 'E:/LandsatDEM/result/aspect_prj.tif'

    gdal.AllRegister()

    data = gdal.Open(DEMFilename, GA_ReadOnly)
    if data is None:
        print('Cannot open this file:' + DEMFilename)
        sys.exit(1)

    dx = 30  # 分辨率

    # 投影变换
    projData = convertProjection(data, LandsatFilename)

    gridNew = projData.ReadAsArray().astype(np.float)

    Sx, Sy = calcFiniteSlopes(gridNew, dx)
    # 坡度计算
    slope = np.arctan(np.sqrt(Sx ** 2 + Sy ** 2)) * 57.29578

    # 坡向计算
    aspect = np.ones([Sx.shape[0], Sx.shape[1]]).astype(np.float32)
    for i in range(Sx.shape[0]):
        for j in range(Sy.shape[1]):
            sx = float(Sx[i, j])
            sy = float(Sy[i, j])
            if (sx == 0.0) & (sy == 0.0):
                aspect[i, j] = -1
            elif sx == 0.0:
                if sy > 0.0:
                    aspect[i, j] = 0.0
                else:
                    aspect[i, j] = 180.0
            elif sy == 0.0:
                if sx > 0.0:
                    aspect[i, j] = 90.0
                else:
                    aspect[i, j] = 270.0
            else:
                aspect[i, j] = float(math.atan2(sy, sx) * 57.29578)
                if aspect[i, j] < 0.0:
                    aspect[i, j] = 90.0 - aspect[i, j]
                elif aspect[i, j] > 90.0:
                    aspect[i, j] = 360.0 - aspect[i, j] + 90.0
                else:
                    aspect[i, j] = 90.0 - aspect[i, j]

    # 输出坡度坡向文件
    driver = gdal.GetDriverByName('GTiff')
    if os.path.exists(slopeFilename):
        os.remove(slopeFilename)
    if os.path.exists(aspectFilename):
        os.remove(aspectFilename)

    ds1 = driver.Create(slopeFilename, slope.shape[1], slope.shape[0], 1, GDT_Float32)
    band = ds1.GetRasterBand(1)
    band.WriteArray(slope, 0, 0)

    ds2 = driver.Create(aspectFilename, aspect.shape[1], aspect.shape[0], 1, GDT_Float32)
    band = ds2.GetRasterBand(1)
    band.WriteArray(aspect, 0, 0)

    del ds1
    del ds2
    data = None
    projData = None

参考：

1.坡向工具的工作原理：http://help.arcgis.com/zh-cn/arcgisdesktop/10.0/help/index.html#//009z000000vp000000

2.坡度的工作原理：http://help.arcgis.com/zh-cn/arcgisdesktop/10.0/help/index.html#//009z000000vz000000

3.http://pangea.stanford.edu/~samuelj/musings/index.html

4.GDAL使用DEM数据计算坡度坡向：https://blog.csdn.net/liminlu0314/article/details/8498985