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Optimisation
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The design of the HLU Tool has been optimised as far as possible and there are no known simple technological enhancements that can be made to significantly improve performance.

It is important therefore to ensure performance is optimised elsewhere wherever possible through user configuration and operation. The following suggestions show some simple approaches to improving performance.

.. index::
	single: Optimisation; GIS

.. _gis_optimisation:

GIS Optimisation
================

GIS Layer Indexing
------------------

Attribute indexes
	Certain functions in the HLU Tool will perform attribute searches on the GIS layer, typically using the **INCID** and/or **TOID** fields. It is, therefore, recommended that individual attribute indexes are created on both of these fields.

Spatial Indexes
	ArcGIS users that frequently zoom in and out using the GIS application whilst the GIS layer is visible, when reviewing or editing using the HLU Tool, then it may be advisable to create one or more spatial indexes on the GIS layer to improve performance when ArcGIS draws the layer. The choice of scales will depend upon the number, size and density of the features in the GIS layer and the preferred working practices of the users (e.g. if they zoom in and out frequently rather than panning).


Creation of ArcGIS Map Document or MapInfo Workspace
----------------------------------------------------

An ArcGIS Map Document (.mxd) or MapInfo Workspace (.wor) must be created for use with the HLU Tool. This should be optimised to ensure maximum performance, which should include the following:

	* Ensure that the .mxd or .wor file only contains one copy of the HLU layer (or one copy of each HLU Layer if split into separate layers).
	* Ensure that the .mxd or .wor is saved at a sensible zoom level such as 1:10,000 scale rather the full extent of the HLU dataset (to save time when opening).
	* Add zoom layering to datasets so that detailed datasets such as the HLU layer and aerial photography are not displayed at smaller scales than necessary. Recommended maximum extents are shown in the table below):

	.. tabularcolumns:: |L|C|C|

	.. table:: Recommended maximum GIS extents

		+------------------------------------+-----------+--------------------+
		|          GIS Application           | HLU layer | Aerial Photography |
		+====================================+===========+====================+
		| ArcGIS (Do not display out beyond) | 1:24,000  | 1:10,000           |
		+------------------------------------+-----------+--------------------+
		| MapInfo (Max Zoom)                 | 7.5 km    | 3 km               |
		+------------------------------------+-----------+--------------------+


Tips for ArcGIS and MapInfo users
---------------------------------

It takes a significant length of time for ArcGIS or MapInfo to draw or query an entire HLU GIS layer, therefore care should be taken when using certain functions. The following tips may help avoid some performance issues:

	* 'Zoom to Selection' is useful for identifying the habitat polygons on the map, however if zoom layering is not used it may take a significant length of time to display the result depending upon the number of polygons selected and their geographical distribution.
	* 'Select by Attributes' performs complex queries and selects the results in the GIS window. If a large number of results are returned, it could take a long time to select the spatial polygons in the GIS.
	* If the GIS layer is taking a long time to draw, pause or cancel the drawing using:

		* ArcGIS - Click the :guilabel:`Pause` button in the bottom left corner of the map window
		* MapInfo – Press the keyboard :kbd:`Esc` key

	* For ArcGIS users, if an ArcMap error occurs when the tool is used, check that the HLU ArcMap Extension is enabled in ArcGIS via **Tools --> Extensions**.  See :ref:`enable_extension` for more details.
	* For ArcGIS users, we strongly recommend that the HLU layer is stored as a file geodatabase or personal geodatabase.

	.. caution::
		The tool will be significantly slower if the HLU layer is stored as a shapefile due to the limitations of the file format.

	* For MapInfo users, ensure the Cosmetic layer is editable when digitising a polygon for a split operation. The polygon will be added to the layer much faster than if the HLU layer was used and also removes the possibility that the polygon is not deleted after the split.


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.. index::
	single: Optimisation; Database

.. _database_optimisation:

Database Optimisation
=====================

Database Type
-------------

The HLU Tool is designed to work with both Microsoft Access and SQL Server databases (other databases are supported but have not been tested).

Microsoft Access is a versatile desktop database application, but does not have the same performance as other database systems. Database management systems such as SQL Server are optimised to handle large data volumes and complex queries, and can be significantly faster.

It is therefore recommend that the HLU Tool is used with SQL Server or a similar database management system as this will improve performance when filtering the database records and updating attribute data.


Local vs. Network Storage
-------------------------

It is important to remember that application performance will depend upon the data transfer speed. Data stored locally on a single computer will provide good performance, but will limit access to the data to a single user. Data stored on a network drive is accessible to all users, but performance will be limited by the speed that the data can be transferred across the network.


Data Management
---------------

Only one copy of the database and the GIS layer should be used to avoid data becoming corrupted. If multiple copies are used e.g. an ArcGIS layer and a MapInfo layer or an Access database and a SQL Server database, changes to one GIS layer or database will not be present on the other, causing a mismatch between the attribute and spatial data.

Habitat data must not be edited directly in either the database or the GIS layer. Any modifications made outside the HLU Tool could cause data corruption particularly if unique identifiers are altered.

However, if additional entries are required in the lookup tables, these may be added to the database directly. It is essential that the structure of these tables is not altered and we recommend that any updates to the data in these tables are carried out solely by the database administrator.


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