Make quick inundation maps in QGIS using Open Data

the purposes of this video is to show how we can flutter DM just based on certain water level relative to the sea level so we'll start with a blank project and first need to locate Benin so we can use the quick map services you can install it from plugins manage and install plugins and you look for quick map services after installing you can find it in the web menu and if you don't have a long list like this you go to settings more services and you get the contributed back now we use the OpenStreetMap as a background over them I was in standard to see a map of the world and we zoom into Benin you're interested only in the certain parts you've got to know they're this and what we do now is change our projection to the local one so we enable on-the-fly projection and you choose UTM zone 31 north and now project is we projected the next step is to download the SRTM digital elevation model therefore you need to SRTM downloader plugin you can also find it in the plugins menu note that to install plugins you always need a working internet connection install the SRTM downloader then you get this button and you can say set to canvas extent to download the elevation data that exists in the area as resumed in - it's always wise to make a local copy so we save it to a folder make a new folder make sure you don't use spaces but underscores select new and then the data will be start there click download and then it has downloaded the DM need to do a bit about visualization so never trust the standard legend that is displayed by gif software always go to properties style in this case we use single band pseudo color and the minimum maximum that are estimated are not right so what we do is you choose the min/max and we use the actual values if I do load you'll see it goes from minus 13 275 meters but in fact we are only interested in the lower part so what I do is clip to canvas and then I click load again and I see it goes to 4 minus 13 to 18 meters we need to choose color ramp choose a new color ramp go to CPT City and choose topography elevation ok and then it's automatically added and that's sufficient for now you see the elevation now similarly we are if we are only interested in the lower parts of the catchment around cotton ooh we can also style it let's focus in this exercise to that we load only the minimum maximum to this part of the image which goes from minus 1 to 10 meters there it is and we see much more contrast in the urbanized area urbanized because this is the of a street map and it gives us the streets but also the building footprints which will be useful later I'll show that later come back to this whole area move the DM so what we can do now is from the DM derive the flood maps we will use the raster calculator and we simply do the DM does this layer less than 1 meter give that one which is true for the rest of the map give zero which is false and save it to our new folder and give it a nice name flood less than one so it's good to add TIF because it's a tiff file and you okay they are this it doesn't look nice yet because we need to do some styling never trust the legend so we go to properties and go to single band pseudo color we only have two values so I just use the plus button and the zero and there's one zero is not flooded and the one is flooded and let's make this one blue so it looks like water then do okay so everything blood blue is flooded during one meter above sea level water level and the rest is not flooded not to make the rest transparent to go to properties transparency and we type here zero and now the zero values will be transparent and you'll see the elevation model through it so everything in blue is flooded and the rest is elevation model or we can even project it here on the map and we can see that one meter we flood these parts of the city we can do the same but then for two meters so let me repeat that raster calculator input is the DM less than 2 meters and I'll do that Slavko to new and call this one flood less than to adopt if save and run it and we can copy this legend clicking write styles copy style and paste it Styles based style and we make the zero transparent it doesn't copy that so we have to type here again zero ok and now we can see the difference this is one meter and this is two meter we see quite an increase in area that is flooded we can visualize this and let's zoom in a bit we go to the plugins and we install new GIS to 3ks that is a plugin to have these 3d views when I click here on this button which then appears you can first indicate the DM always make sure you have the DM layer here which is this n06 style increase the resolution to get very nice results and we increase here also the resolution to 400% because the area is quite flat it's very important to exaggerate so let's put it on 10 when I click run it will calculate and the 3d view which will open in a browser it's very nice thing because you can use it offline you can take it with you files if you want to demo to other people you don't have internet at hand you ever see the area again and see parts that are flooded and what we can also do is put the DM in the background here it is with blue still the flooded parts you can repeat the visualization in fairness and then you can see that the blue parts are really in the depressions now we saw these building footprints here on the map and the nice thing compared to Google Maps is that these are real factors so we can use them and we know that it's not complete but let's say we are interested in in a certain area and we can see what the effect is of flooding so let's focus on the area here on the north let's remove the flood maps and let's download the building footprints we see that are already buildings built in the water and here for we need another plugin it's called the quick osm plug-in its quick always-on plug-in with that one we can do queries from the OSM data set and download what we need so in our case building footprints but you could also get the water related factors of the roads etc so you will get this button when I click there I can choose here he fell you from of the street map in our case it will be building and the value should be yes so we get all the buildings and I use it only in the extent of the map so I run the colleague ons of this area and there it is so let's make them a bit different color which is red okay and so what we have is the building footprints but I also would like to have is this flood extent as a polygon so I'm going to export this one to a polygon you do it by raster conversion polygon eyes and choose the right folder and I call it flood two meters not SHP and there we go okay okay close now this one exported everything so the zero value so what we need to do in the attribute table we first select all the ones so array two fields and values the end is the name of the column if I double-click equals type one to select now we have all the flood area selected I go to editing mode I invert the selection and I'm going to remove everything what is not flooded from this there it is so we now only have the flooded part and I'm also going to change this value to two so we know it's two meters so we can put it on top of the DM at 2 meter level the absolute height so TN equals to update oh now it's 2 meters I save it and there it is and let's make it blue and let's now see how this will look in two and a half D when you have the building footprints so I select the buildings to be plotted and these buildings let's give them a fixed value above the DM let's assume that they are five meters high and also the floods but a flood we give them value based on the TN which is 2 meters multiplied by 1 and that's relative to know TEM or absolute value in our case it is absolute value then I click run so here we can see the flooded zones in the depressions we can use the layer with inundation also to calculate the flood area and if we know the elevation of the surface we can also calculate the volume of water that is needed to flood the area so we have the de m which is the surface area we intersect with a certain level of flooding that creates area so we know the elevation difference and the area that will give us the volume

Channel: Hans van der Kwast

Views: 14,717

Rating: 4.8942733 out of 5

Keywords: QGIS, SRTM, OpenStreetMap

Id: QXQ92oVua3s

Length: 16min 32sec (992 seconds)

Published: Wed Aug 15 2018