如何从DEM(数字高程模型)中提取特定值? [英] How to extract specific values from a DEM (digital elevation model)?

问题描述

我正在尝试使用开放数据(避免像Google这样的许可限制)来计算远足路线的海拔数据.

I'm trying to calculate elevation data for hiking routes, using open data (avoiding licensing constraints like Google).

我能够读取我所在国家/地区的公开DEM(分辨率为10米) 使用readGDAL(来自RGDAL软件包)和proj4string(mygrid)给我:

I was able to read a public DEM of my country (with a 10-metres resolution) using readGDAL (from package RGDAL), and proj4string(mygrid) gives me:

"+proj=utm +zone=32 +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0"

.asc文件的开头是:

The beginning of the .asc file is:

ncols         9000 
nrows         8884 
xllcorner     323256,181155 
yllcorner     4879269,74709 
cellsize      10 
NODATA_value  -9999 
978 998 1005 1008 1012 1016 1020 1025 ..... 
..... 
..... 400 Megabytes of elevation values .... 
..... 

我需要做的就是从此网格中获取路线特定节点的高程数据，以便能够 计算高程增益，负斜率，最小/最大高度...

All that I need is to pick up from this grid the elevation data for the specific nodes of a route, so to be able to calculate elevation gain, negative slope, min/max altitude...

我使用不错的OSMAR包从OpenStreetMap带来了路线数据，所以我路线的数据表是这样的:

I bring the routes data from OpenStreetMap, using the nice package OSMAR, so the data table of my route is something like this:

    RouteId NodeId  lat         lon
1   -13828  -8754   45.36743    7.753664
2   -13828  -8756   45.36762    7.753878
3   -13828  -8758   45.36782    7.754344
4   -13828  -8760   45.36794    7.754541
....

但是我不知道如何在DEM坐标参考系统中变换纬度/经度坐标，然后如何带来相应的网格值(对最近点进行平均?)

But I've no idea how to transform the latitude/longitude coordinates in the DEM coordinate reference system, and then how to bring the corresponding grid values (doing a sort of average of the nearest points?)

我发现所有使用谷歌搜索的文档都将渲染网格图，而不是从中提取值.

All the documentation I've found googling it is about to render grid maps, not to extract values from them.

任何帮助将不胜感激！

干杯，MB

P.S.第二个问题应该是: 具有多个网格拼贴，如果一条路线跨越两个或多个拼贴，该怎么办?将它们合并，同时引用两者..."

P.S. The second question should be: "Having several grid-tiles, what could I do if a route is across two or more tiles? Merge them, reference both..."

推荐答案

请勿转换DEM.改变你的观点.您的DEM投影在常规网格上.如果您将其重新投影到另一个项目中，则可能不会.而是从OSM数据中得出空间点:

Don't transform the DEM. Transform your points. Your DEM is projected over a regular grid. If you re-project it into another it may not be. Instead, make spatial points from your OSM data:

require(sp); coordinates(my.OSM.points) <- long + lat

然后将它们转换为DEM的坐标参考系统(请注意，您可能需要首先设置点的CRS.因此，您可以这样做:

Then transform them to the coordinate reference system of the DEM (note you may need to set the CRS of the points first. So you can do this:

#  Set pojection information for points
proj4string(my.OSM.points) <- CRS("+proj=longlat +datum=WGS84 +ellps=WGS84")

#  Transform them
new.points <- spTransform( my.OSM.points , proj4string( mygrid ) )

然后看看使用sp::over或raster::extract来获取点位置的DEM值.

Then look at using sp::over or raster::extract to get the values of the DEM at point locations.

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