Data preparation centralized / decentralized question¶
What do you need?
Dowload QGIS from the official website and installed it following the installation guide.
Get started with QGIS using tutorials (https://docs.qgis.org/3.40/en/docs/training_manual/index.html for instance)
Identify the available GIS data in your context and dowload them (DEM, roads and buildings): government platform for data provision, existing QGIS plugin.
Quantify/estimate the population to connect
This flowchart aims to guide you in preparing your data prior to create scenarios on your own study area (next flowchart).
It is interactive: you can zoom in and click on certain blocks to be redirected to the corresponding sections in the documentation.
flowchart TB
A1["I open a QGIS project"]
A2[<a href="https://elan-gis.org/en/petite_anse.html#basemap">I display the OpenStreetMap basemap</a>]
A3["Do I know where to find DEM in my local context (min resolution 12.5m) ?"]
A1 --> A2 --> A3
B1["I load my DEM into my project"]
B2[Am I in French context?]
B3[<a href="https://elan-gis.org/en/petite_anse.html#rge-alti">I download 5m DEM on *RGE Alti*</a>]
B4[<a href="https://elan-gis.org/en/preparation.html#alaska-edu">I search on *alaska.edu* to find 12.5m DEM</a>]
A3 -- yes --> B1
A3 -- no --> B2
B2 -- yes --> B3 --> B1
B2 -- no --> B4 --> B1
B5["The detected CRS is correct (Predefined and not Custom)?"]
B6[I correct by selecting the corresponding QGIS predefined CRS]
B7[<a href="https://elan-gis.org/en/petite_anse.html#scr-projet">I define the project CRS from DEM layer</a>]
B8[Is my DEM made of only 1 tile?]
B1 --> B5 -- yes --> B7 --> B8
B5 -- no --> B6 --> B7
C1[My DEM greatly exceeds the study area?]
C2[<a href="https://elan-gis.org/en/petite_anse.html#fusion">I use the GDAL *Merge* tool</a>]
C3[Does my DEM have missing pixels?]
C3a[Missing pixels are few or/and far enough from the area you want to connect?]
C3b[I look for another DEM tile and I repeat previous steps OR I extrapolate values of the missing pixels]
B8 -- yes --> C1
B8 -- no --> C2 --> C1
C1 -- no --> C3
C4["I use the QGIS *Clip to extent* tool"]
C5["I check the detected CRS is correct (Predefined and not Custom)." I correct it if necessary.]
C6[<a href="https://elan-gis.org/en/petite_anse.html#scr-projet">I define the project CRS from my clipped DEM layer</a>]
C7[My DEM is ready! <a href="https://elan-gis.org/en/petite_anse.html#symbologie-mnt">I can adjust its symbology to better see the contrasts when I zoom in</a>]
C1 -- yes --> C4 --> C5 --> C6 --> C3
C3 -- yes --> C3a -- yes --> C7
C3a -- no --> C3b --> C7
C3 -- no --> C7
D1["Do I know where to find my vector layers (roads and buildings) in my local context?"]
D2[Am I in French context?]
D3[<a href="https://elan-gis.org/en/petite_anse.html#ign-topo">I use *BD TOPO extractor* to retrieve roads and buildings</a>]
D4[Is my area in the southern hemisphere?]
D5[<a href="https://elan-gis.org/en/preparation.html#open-buildings">I download my buildings on *Open Buildings*</a>]
D6[<a href="https://elan-gis.org/en/petite_anse.html#creer-zone">I create a polygon layer to define the study area</a>]
D7["Do my vector layers accurately transcribe the field reality (buildings to be connected and possible roads)?"]
D8[<a href="https://elan-gis.org/en/petite_anse.html#creer-zone">I create a polygon layer to define the study area</a>]
C7 --> D1
D1 -- yes --> D7
D1 -- no --> D2
D2 -- yes --> D6 --> D3 --> D7
D2 -- no --> D4
E1[<a href="https://elan-gis.org/en/petite_anse.html#creer-zone">I create a polygon layer to define the study area</a>]
E2[<a href="https://elan-gis.org/en/preparation.html#routes">I use the Elan *Roads and buildings* module to get</a>]
E3[my roads]
E4[my roads and my buildings]
D4 -- yes --> D5 --> D8 --> E2 --> E3 --> D7
D4 -- no --> E1 --> E2 --> E4 --> D7
E5[<a href="https://elan-gis.org/en/petite_anse.html#edition-manuelle">I edit manually my layers: addition/deletion of roads and buildings eventually based on satellite images</a>]
E6[Are all my layers in the same CRS?]
D7 -- yes --> E6
D7 -- no --> E5 --> E6
F1[Do I know where possible discharge points are?]
F2[I project my vector layers in this CRS using QGIS *Reproject layer* tool]
F3[<a href="https://elan-gis.org/en/petite_anse.html#set-exutoires">I create a point layer in the project CRS with all possible discharge points</a>]
F4[I go on : Elan will automatically position a discharge point on the lowest pixel of my area]
F5["I check if all my features (roads, buildings, discharge points) are on my DEM tile"]
F6["Do I have an estimation of the population to connect (total number of inhabitants)?"]
E6 -- yes --> F1
E6 -- no --> F2 --> F1
F1 -- yes --> F3 --> F5
F1 -- non --> F4 --> F5
F5 --> F6
G1["Does a uniform distribution of the population within the area suits me (same number of inhabitants/building)?"]
G2[I do some research]
G3[I estimate a mean number of inhabitants/building based on the number of buildings I have in my layer]
G4[<a href="https://elan-gis.org/en/preparation.html#uniforme">I use the Elan *Population uniform distribution module*</a>]
G5[I get a building layer as centroids with a population attribute]
G6["Do I want to pre-size only main arteries of the sewer network (faster)?"]
G7[<a href="https://elan-gis.org/en/preparation.html#projection">I use the Elan *Snap on roads* module</a>]
F6 -- yes --> G1
F6 -- no --> G2 --> G1
G1 -- yes --> G3 --> G4 --> G5
H1[<a href="https://elan-gis.org/en/preparation.html#surfacique">I use the Elan *Population areametric distribution* module to get a number of inhabitants adjusted to buildings footprint</a>]
H2["I check if my centroids layer is in the correct CRS (project CRS)"]
H3[Do I know my sizing constraints? <br/> - average production of wastewater/day/person <br/> - min and max sewer depths <br/> - possible diameters]
H4[I do research / make hypotheses]
H5[I am ready to create a scenario !]
G1 -- no --> H1 --> G5 --> G6
G6 -- yes --> G7 --> H2
G6 -- no --> H2 --> H3
H3 -- yes --> H5
H3 -- no --> H4 --> H5