QGIS Terrain Analysis: hillshade, slope, aspect (Version 3.x) — Transcript

Full transcript

1. 0:00[Music]
2. 0:01alright guys so in the next video I do I
3. 0:04want to show you how you can make a 3d
4. 0:06map in QGIS so in this video what we'll
5. 0:09do is we'll get prepped for that by
6. 0:12doing some raster terrain analysis that
7. 0:14we'll prep us to make a 3d map in the
8. 0:17next video so stick around for some
9. 0:20terrain analysis that will prep you for
10. 0:23the 3d mapping alright so the first
11. 0:26thing I want to do is I'm going to add
12. 0:28in a digital elevation model and I'm
13. 0:31gonna use this de and we've been using
14. 0:35for a few of our analyses so far that's
15. 0:39not the one I want I have one down here
16. 0:41this one this is going to be clipped to
17. 0:43a watershed boundary so let me go back
18. 0:45into my layers and we'll just remove
19. 0:48this here and then I'm gonna just zoom
20. 0:53to this layer so it fills up the screen
21. 0:56ok so here you can see I have a digital
22. 0:58elevation model of a watershed and this
23. 1:01is what I use in a lot of my videos this
24. 1:02is the temple for watershed it's up in
25. 1:04northern Utah and so what these pixel
26. 1:08values represent is elevation above sea
27. 1:10level in meters and so this this area
28. 1:14here is 181 meters above sea level and
29. 1:16that Peter work 2630 metres above sea
30. 1:19level okay so you can do some really
31. 1:22cool stuff with a digital elevation
32. 1:27models of DDMS and one of them is to
33. 1:30create a hill shade and so if you're not
34. 1:32familiar with the hill shade a hill
35. 1:33shade just kind of gives you a photo of
36. 1:37whether it's a part of you looks like
37. 1:38and so we'll go ahead and run it and
38. 1:41when you see you might get an idea of
39. 1:42what it is and then I'll talk about why
40. 1:44we might want to Hill shade so the first
41. 1:46thing to run a hill shade we can come
42. 1:48into raster here and we can come and do
43. 1:50analysis and we have this hill shade
44. 1:55button and it's in the G Dow algorithm
45. 1:58box so I'm gonna go ahead and I'm gonna
46. 2:02click on Hill shade and you can see my
47. 2:05input is already selected EEM and I only
48. 2:08have one band my band number you can
49. 2:10leave these defaults the same if you're
50. 2:12in this
51. 2:13hemisphere you might want to change the
52. 2:14azimuth of light so if you're otherwise
53. 2:19the shading might look backwards so you
54. 2:21can run if your shading looks off you
55. 2:23can change it okay and so what I'm gonna
56. 2:26do here is I'm going to come down to my
57. 2:28save file I'm gonna change this I'm
58. 2:31gonna just save it to a file and this
59. 2:34will pop up in my dialog here and so
60. 2:37let's go back to here I think want to go
61. 2:41back into my temp data and I think we
62. 2:45had my outputs here here's my over my
63. 2:49TEM tems I already have a heal shade
64. 2:51we'll just name this hillshade - you can
65. 2:54see what it looks like and I'll click
66. 2:56Save and once we have that we can go
67. 3:00ahead and we can click run this is gonna
68. 3:03process the algorithm which might take
69. 3:05just a minute the bigger your raster is
70. 3:07the longer this takes so we're finished
71. 3:09I can close this and you can see that
72. 3:11it's added this Gila shade in here okay
73. 3:15so you can see there's a canyon here and
74. 3:17there's a big canyon here we've got a
75. 3:19little more mild topography out of this
76. 3:21area
77. 3:23now I'll show you something real cool
78. 3:24with the hillshade so what I'm gonna do
79. 3:27is I'm gonna take the steel shade and
80. 3:28drag it below my de m and this can
81. 3:31really help our symbology out so I'll
82. 3:33drag it below here I'm gonna change my
83. 3:35de M symbology now so I'm gonna open
84. 3:38this up by double clicking or
85. 3:40right-click and go to properties come to
86. 3:42symbology and the first thing I'll do is
87. 3:45I'm going to change this to single band
88. 3:47Steudle color and then I'm going to use
89. 3:51this color ramp here that I've developed
90. 3:52myself which is my elevation Green Bay
91. 3:55on white color band color ramp and if
92. 3:58you want to see how to make this color
93. 4:00ramp I have a video that I'll link in
94. 4:03the cards here and in the description
95. 4:04below that will tell you how to make a
96. 4:06custom color ramp in QGIS so I'm going
97. 4:10to do that and then the next thing we
98. 4:12becoming transparent see I'm gonna make
99. 4:14the global opacity I'm just going to
100. 4:17make a 60% okay
101. 4:23and I'll come over here like a apply all
102. 4:25those things and now you can see that
103. 4:28that hillshade shows through and you
104. 4:30have the elevation values over the top
105. 4:32of this but you now also have that that
106. 4:37hill shade underneath that kind of makes
107. 4:38that topography pop out so that's a cool
108. 4:40little symbology Creek you can you can
109. 4:42try and that's what we'll use we use
110. 4:45this DDM and the hill shade to make a
111. 4:47really cool-looking 3d map in the next
112. 4:49video but while we're on top I want to
113. 4:51go through some more terrain analysis
114. 4:53with you and so go back into raster and
115. 4:59come back into my analysis tools and two
116. 5:02more common terrain analysis tools that
117. 5:08we use our slope and aspect and so we'll
118. 5:12just go through these little quick so
119. 5:14let's run the slope this is also going
120. 5:16to take a DDM input which I have here
121. 5:18we're gonna give it a first band and
122. 5:20we're just gonna leave the ratio to say
123. 5:22we want the ratio to be one two ones we
124. 5:24don't want any stretching there and so
125. 5:29slope express the percent instead of
126. 5:32degrees so there are two ways to do
127. 5:34slope there are degrees and percentages
128. 5:36okay you're probably the most familiar
129. 5:38for the centage is maybe a big reason
130. 5:40she's gonna give you the angle between 0
131. 5:42and 90 and vertical will do this just
132. 5:45degrees so we're gonna get values that
133. 5:47range from 0 to infinity I'm for doing a
134. 5:50percent so we'll leave it like that this
135. 5:54other option is to compute the edges so
136. 5:56the slope algorithm uses a neighborhood
137. 5:58search and I'll show you what I mean by
138. 6:00that I'm going to close this real quick
139. 6:02I'm gonna zoom in on a really close to
140. 6:06an area my pixel later my map so you'll
141. 6:09notice here if I turn my Hill shade off
142. 6:12then turn it back on you can see the
143. 6:15hill shade not get computed for these
144. 6:17edge cells and that's because of most of
145. 6:20these raster analysis tools use a 3x3
146. 6:23moving window to do a calculation and so
147. 6:25what that looks like they have one cell
148. 6:27here one cell here one cell here so
149. 6:29there's the Leros and i have a cell here
150. 6:32a cell here and a cell here there's
151. 6:33three columns and so we'll take this
152. 6:35area in here
153. 6:37and we'll perform a calculation that
154. 6:39involves all nine of those cells to get
155. 6:42a value for the middle cell and if there
156. 6:45are no data values so like this cell
157. 6:47right here there are no data values for
158. 6:49three of those cells and so it won't
159. 6:51perform the calculation there which
160. 6:53often isn't a big deal sometimes it is
161. 6:55so with the slope let's go back to our
162. 7:01raster analysis and go back to slope
163. 7:03this is just thing it will compute the
164. 7:06edges even if there aren't nine sales
165. 7:10with theta in then you can compute the
166. 7:11edges but you might have some anomalies
167. 7:14at those edges because of the no data
168. 7:15values so just be aware of that I'm not
169. 7:17going to compute the edges I'm gonna
170. 7:19save to a temporary file I doser I'm
171. 7:22gonna save to a file and we'll save it
172. 7:25in the same spot here and I'll call this
173. 7:26slope and we'll call it P ER so we know
174. 7:33it's percent slope but not click Save
175. 7:35and then here I'll click run once again
176. 7:40here's my output so I can close this so
177. 7:46you can see my slope values here range
178. 7:51from 0 to 53 percent okay or maybe I did
179. 7:57this in degrees I'll to go back and
180. 7:59check what the default is sorry guys I
181. 8:01may have just led you straight a little
182. 8:02bit there let's go back to slope okay so
183. 8:06this is in degrees sorry I named that
184. 8:08file wrong
185. 8:10but this is so this is in degrees slope
186. 8:13and so 0 to 53 so let's go ahead and we
187. 8:16can go change the symbology on this I
188. 8:19generally like to make my slope kind of
189. 8:31in bright colors here so maybe the green
190. 8:35red yellow might be a good one for this
191. 8:37and I want my red to be high slope so
192. 8:39I'm gonna come in and I'm gonna invert
193. 8:41the color ramp and it won't click OK and
194. 8:45so there you can see that those red
195. 8:47areas show you where the high slopes are
196. 8:50you can do the same thing with this is
197. 8:51with the de M on a hill shade you can
198. 8:53come in and we can change our
199. 8:56transparency again so I'll make this 60%
200. 9:02and then you can kind of get an idea of
201. 9:04where those slope values occur all right
202. 9:07so let's go ahead and do the aspect if
203. 9:12you're not familiar with aspect what
204. 9:14aspect is if sheet gives you the
205. 9:15direction of slope is facing and we can
206. 9:20get that a little bit with our hill
207. 9:22shade right we can see this slope faces
208. 9:24south this one faces north you know this
209. 9:27one faces south west this one faces
210. 9:30north west so we can kind of come in and
211. 9:32look at that but if you do aspect
212. 9:34they'll give us a numerical value in
213. 9:36degrees in the direction of slope faces
214. 9:39so once again all we need here is our de
215. 9:42M input our first band we can do a
216. 9:46trigonometric angle instead of azimuth
217. 9:48we want azimuth we can get our flat area
218. 9:52so areas are completely flat we can make
219. 9:54them so you don't stead of negative
220. 9:55instead of no data so this is a no data
221. 9:57value I'll make those zero we can do the
222. 10:00same thing with compute edges but I'm
223. 10:01gonna just leave that default so let's
224. 10:04come down here and we'll save to a file
225. 10:07and we'll name this aspect and we'll
226. 10:10click Save whole click run and here's
227. 10:15our output okay so now we're done all
228. 10:18right
229. 10:19and so these areas are flat there are no
230. 10:21data values over here but there are also
231. 10:23flat values so let's just go ahead and
232. 10:26take a look it should be zero there okay
233. 10:30and that's a low value all right so
234. 10:34let's go ahead and let's break these up
235. 10:39a little differently so let's go to
236. 10:40symbology and we'll do single band
237. 10:43pseudo color again this time if we
238. 10:49divide this up into like a equal classes
239. 10:52we should be able to see
240. 10:59[Music]
241. 11:04the directions a little better so let's
242. 11:06do eight classes click classify okay so
243. 11:14those are broken up they're pretty
244. 11:15evenly and I'm gonna change my color
245. 11:21gonna eyes not be sure what I want this
246. 11:22to be yet but let's take a look you try
247. 11:27like the inferno color so that gives us
248. 11:30no data those will be slopes the face to
249. 11:34the northeast okay okay let's you bad
250. 11:43try it'd be nice that this was we could
251. 11:46adjust these manually but we'll just
252. 11:48leave them they'll be roughly out okay
253. 11:49so looks like okay here alright so now
254. 11:56you can see which direction these slows
255. 11:58face so values here is gonna be no data
256. 12:00and then we get these dark values
257. 12:02they're gonna face to the northeast
258. 12:04these values over here we're gonna face
259. 12:09to the north-west okay these values are
260. 12:13gonna be facing to the south right in
261. 12:17here and so that you're ready can just
262. 12:19kind of see the direction those slopes
263. 12:20are facing alright so those are some of
264. 12:23the three analysis tools you can use
265. 12:26here in QGIS pretty easy to use pretty
266. 12:30intuitive to use if you have questions
267. 12:32about them please leave a comment down
268. 12:33below I'd love to get to those questions
269. 12:35if you have suggestions for any other
270. 12:37tutorials let me know and next week what
271. 12:40we use is we use the de m and the
272. 12:43hillshade to create a 3d map in QGIS and
273. 12:48it actually looks really cool and it's
274. 12:50really easy to do so stick around
275. 12:52subscribe so you're notified when next
276. 12:54week's video comes out and you'll be
277. 12:56able to see how to make that 3d map
278. 12:58thanks for watching guys hope you have a
279. 13:00good day if you have questions again
280. 13:02please let me know