> For the complete documentation index, see [llms.txt](https://guides.geospatial.bas.ac.uk/ops_training/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://guides.geospatial.bas.ac.uk/ops_training/gis-technologies-at-bas/web-gis/layer-structure.md).

# Layer Structure

GIS systems use a layer structure to organise the data.  Likewise, Web mapping also arranges our base data, and then layers other contextual data over the top.  

## Base Layers

Base layers are normally made up of **raster data**, such as **satellite imagery**, **digital elevation models** (DEMS), **hillshades** or a combination of both **raster and vector data**.  

You can **toggle** between base layers as you use the map.  Examples of base layers available in Operations GIS are listed below: 

|                    Layer | Description                                                                                                                                                                       |
| -----------------------: | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Hillshade and Bathymetry | Antarctic hillshade and bathymetry south of 60°S                                                                                                                                  |
|              LIMA mosaic | Landsat Image Mosaic of Antarctica (LIMA). True colour (bands 3, 2, 1), pan-sharpened. Imagery dated 1999-2003. ([learn more here](https://lima.usgs.gov/access.php))             |
|                 REMA DEM | Reference Elevation Model of Antarctica, a Digital Surface Model (DSM) at 8-meter spatial resolution from the Polar Geospatial Center ([PGC](https://www.pgc.umn.edu/data/rema/)) |

![Operations GIS with base layers highlighted.  As you can see, Radarsat RAMP HH is toggled here.](/files/-MW0Cs6ea4sZY7758r5M)

## Reference Layers

The rest of the layers are called Reference Layers, and contain a mixture of Topographic and Operations data.  

### Topographic Data

Topographic data layers are sourced from the [Antarctic Digital Database](https://add.scar.org/). 

{% hint style="warning" %}
The ADD is updated twice a year and is available for download [here](https://add.scar.org/).\
At **Rothera**, the data is updated at the **start of each season.**  
{% endhint %}

![Operations GIS, highlighting the topographic data layer group](/files/-MW0EcXB-e2qtb0dCM4e)

### Operations Data 

This data is split into **Static Data**, which is useful reference data that is relatively fixed, and **Season Data**, which is updated annually.  

#### Static Data

Examples of static operations data include station-specific data such as Rothera building layout, the Dash-7 PNR radii from airfields, or multi-year tractor traverse routes.

![Operations GIS, highlighting the static data.  Notice we have toggled the "Primary Routes and Waypoints" data layer here.  ](/files/-MW0GorLzzODts0JWaT7)

#### Season Data

Season data is updated annually, and usually includes: 

* Waypoints, Depots and Network routes from the Air Operations Planning Maps
* Science Sites sourced from the Field Operations Manager

![Operations GIS, highlighting the season-specific data group.](/files/-MW0JPAi5LXv4F6qazPC)