The Importance of a Basemap
By Sandra Schloen, June 2016
Updated May 2017
A basemap represents the geographic canvas onto which other geospatially aware items are drawn. It is an image that has been georeferenced -- assigned coordinates in physical space -- within some chosen coordinate system. A basemap might be a raster image, such as a tif file, or it may be a web-based map provided by Esri's web services. See the ArcGIS documentation for a list of supported raster types.
Basemaps within OCHRE
Every Map View in OCHRE requires a basemap. The coordinate system of the supplied basemap determines the coordinate system of the current OCHRE Map View. All other items in the view must have been georeferenced within the same coordinate system as the basemap, or they will be re-projected on-the-fly as they are added to the View.
The basemap determines both the extent and the resolution of the Map View. Items can be placed on the Map only within the geographic boundaries defined by the basemap. Similarly, the zooming capabilities of the Map in OCHRE is limited to the resolution provided by the basemap; that is, the basemap image will not pixelate in order to zoom in beyond its natural resolution. For example, in ArcMap you can right-click a Layer in the Layer pane (on the left) and "Zoom to raster resolution." This will illustrate the maximum resolution to which OCHRE will be able to zoom.
A basemap represented by a supported raster image is added to OCHRE as a regular Resource item with its Type set to "geospatial."
Important! Any georeferenced raster image used as a basemap in OCHRE MUST have a defined spatial reference. That is, the coordinate system used to plot the image in space must be explicitly set; it cannot be "undefined." For example, if viewing a raster image in ArcMap, right-click the Layer that represents the image to view its properties. The Source tab of the Layer Properties must show a properly assigned Spatial Reference (scroll past the other Raster Information to view the Spatial Reference properties).
Image assigned to the Israel Transverse Mercator Coordinate System
Map Options
The Resources Inbox of the OCHRE project is used to identify the default basemap to be used by any Map View for that project. That is, in the absence of any other options, this is the basemap that OCHRE will use. Make sure the Resources Inbox is available by selecting the Resources category in the navigation pane and checking ON the option Show Inbox even if empty. Refresh the Resources category (using the swirl button on the main toolbar) and the Inbox should appear.
Go to the Map Options tab of the Resource Inbox and link in a Resource item--one that has been set up in OCHRE as a georeferenced raster image (e.g. tif) of Type "geospatial"--to be used as the Default basemap,
Alternatively, you can select one of the basemaps provided by Esri and made available through OCHRE. Note that the Esri basemaps all use the common latitude/longitude-based world coordinate system and so are mutually compatible.
The basemap identified on the Map Options of the Resource Inbox is the one that is used as the backdrop for Map View.
Use the Set default extent option to target a specific geographic area. The basemap will be presented in a View. Use the pan and zoom controls to position the map so that the desired region is in view. Upon clicking the Set default extent button on the View menu, OCHRE will capture the coordinates of the required extent and save those for future reference.
The Map Options tab is available on all hierarchies and sets. By establishing options at the level of a hierarchy or set you override the default settings on the Resources Inbox. Views of items within such a hierarchy or set will be displayed on the basemap provided by that hierarchy or set, if any. In the event that no basemap is specified at the level of the hierarchy or set, the basemap identified on the Map Options of the Resource Inbox will be used.
Other Basemap Considerations
- The basemap should be provided at an appropriate resolution. For a regional study showing vegetation zones, for example, you would use a different basemap than you would for a close up view of a 10 x 10 m square of excavation delineating outlines of walls and pits. The overall resolution of the basemap (e.g. raster image) determines the extent to which you can zoom in on the map.
- Geo-referenced raster images must be available to OCHRE in the local file system if being used in Offline mode. Maps fetched from online services (e.g. ArcGIS Online) cannot be used offline. If raster images are needed for a View in OCHRE, and if these images are stored on a server (that is, not locally), OCHRE will download the needed images behind-the-scenes to a temporary folder on the local computer and cache them there so that they are accessible to the mapping viewer. To clear this cache, go to the Offline tab of your User account and click the Clear button there (or just leave the cache alone for faster viewing in subsequent sessions).
Local Basemaps
As mentioned above, OCHRE needs a project-level basemap specified on the Resource Inbox to use as the backdrop for Map View. But in the case of an archaeological project, for example, a basemap that covers the full extent of the site at an appropriately detailed resolution would likely be impractically huge. OCHRE's Map View allows you to attach a more local basemap to any hierarchy, using the Map Options of that hierarchy. Simply link in a raster image Resource as the basemap of a hierarchy and it will be available in Map View. The Information button in Map View will toggle on additional icons that identify the geospatial character of the corresponding item.
Here we see that the Northern Lower Town and the Eastern Citadel regions at Zincirli have local basemaps available. Select the hierarchy item, then click the "Zoom to the extent of the currently selected item" button, highlighted below, and OCHRE will swap in the local basemap identified for that hierarchy. From that point forward, the "Zoom to full extent" and "Zoom to previous extent" buttons apply to the new local extent. To revert to the original project-level basemap, select the Project item at the top of the navigation pane, and click either the "Zoom to full extent" or the "Zoom to the extent of the currently selected item" buttons.
