How to access the AIMS eReefs data using OPeNDAP and ArcMap

OPeNDAP stands for "Open-source Project for a Network Data Access Protocol" (see https://www.opendap.org/about) and can be used to access remote NetCDF files. The AIMS eReefs THREDDS server provides access via OPeNDAP for all NetCDF files produced by the AIMS eReefs system. In this tutorial we will use ArcMap to render the data, but you can also have a look at our tutorials How to plot AIMS eReefs data with R and How to plot AIMS eReefs data with Python for a more programatically approach. More clients can be found on the OPeNDAP clients website.

Should I download the NetCDF files or use OPeNDAP to access the data?

The answer to this question depends mainly on the use-case. In general, the use of OPeNDAP is slower and more un-responsive but it is a good way to explore the data before downloading it (NetCDF files can be very large, sometimes multiple GB). When you exactly know which file(s) you need, it is recommended to download the file(s) (see How to open a NetCDF file with ArcMap and QGIS) as this will make working with the file(s) faster.

Finding the OPeNDAP URL

Open the AIMS eReefs THREDDS server website and select the link Data derived from the eReefs model.
On the next page select the link to the required model
1. GBR4_H2p0_B3p1_Cq3P_Dhnd := eReefs BioGeoChemical model version 3.1 (4 km resolution) - source catchments using Pre-Industrial catchment conditions
2. GBR4_H2p0_B3p1_Cq3R_Dhnd := eReefs BioGeoChemical model version 3.1 (4 km resolution) - source catchments using Hindcast catchment conditions (anthropogenic loads reduced according to the percentage reductions of DIN, PN, PP and TSS specified in the Reef 2050 Water Quality Improvement Plan 2017-2022)
3. GBR4_H2p0_B3p1_Cq3b_Dhnd := eReefs BioGeoChemical model version 3.1 (4 km resolution) - source catchments using Baseline catchment conditions.
4. gbr1_2.0 := eReefs Hydrodynamic model version 2.0 (1 km resolution)
5. gbr4_v2 := eReefs Hydrodynamic model version 2.0 (4 km resolution)
Each model has its derived output grouped into folders with the names indicating the time steps and aggregation in a file. For example a folder with the name"daily-monthly" contains "monthly" files with "daily" data while a folder with the name "annual-annual" contains "annual" files with "annual" data. Select the link to the folder of interest.
In the list of NetCDF files, select the one of interest. The file names describe the data they contain with the structure "EREEFS_organisations_model_timestep-aggregation-date.nc". For example the file "EREEFS_AIMS-CSIRO_gbr4_v2_hydro_daily-monthly-2010-09.nc" can be broken down into:
"AIMS-CSIRO" := Reference to the organisations AIMS and CSIRO
"gbr4_v2_hydro" := eReefs Hydrodynamic model version 2.0 (4 km resolution)
"daily" := "daily" data
"monthly" := one month of data
"2010-09" := data for September 2010
On the single file page in the section "Access" click the link "OPENDAP"
On the next page copy the value of the input field "Data URL". This is the URL for opening the file in a GIS tool.

Access the data with ArcMap

For this step-by-step tutorial ArcMap version 10.6.1 was used.

Start ArcMap.
Select the tool "Make OPeNDAP Raster Layer" by either using the "Search" or expanding the branch "Multidimension Tools" in the "ArcToolbox".

In the popup window

For the input field "Input OPeNDAP URL" paste the URL copied in the previous section ("Finding the OPeNDAP URL").
Click the Variable drop-down arrow and choose a variable from the list.
Leave the default values for "X Dimension" and "Y Dimension".
Enter a layer name in the input field "Output Raster Layer".
Optionally, adjust the "Extent" values
Optionally, to create a multiband raster, click the Band Dimension drop-down arrow and choose a dimension from the list.
Enter "time" in the "Dimension Values (optional)" field and click the "+" (plus) button on the right side.
The dimension "time" should have been added to the list below the field. Now select a "Start Value" and a "End Value" by clicking on the field in the list.
Enter "k" in the "Dimension Values (optional)" field and click the "+" (plus) button on the right side. The dimension "k" represents the depths dimension.

The dimension "k" should have been added to the list below the field. Now select a "Start Value" and a "End Value" by clicking on the field in the list.
Here is an overview of the values for this dimension depending on the selected model:

*Mapping of dimension k to depth value in meters*
Index (k)	Hydrodynamic 1km model	Hydrodynamic and BioGeoChemical 4km model
1	-140.00 m	-145.00 m
2	-120.00 m	-120.00 m
3	-103.00 m	-103.00 m
4	-88.00 m	-88.00 m
5	-73.00 m	-73.00 m
6	-60.00 m	-60.00 m
7	-49.00 m	-49.00 m
8	-39.50 m	-39.50 m
9	-31.00 m	-31.00 m
10	-24.00 m	-23.75 m
11	-18.00 m	-17.75 m
12	-13.00 m	-12.75 m
13	-9.00 m	-8.80 m
14	-5.25 m	-5.55 m
15	-2.35 m	-3.00 m
16	-0.50 m	-1.50 m
17	n/a	-0.50 m

Click "OK" to close the window.

Access data with OPeNDAP in ArcMap - screenshot 1

After processing the input the layer is shown.