Below are the responses to some of the queries which often arise when clients have been supplied with UAV data. If your query is not covered in the list then please call Claire Lush on 01874 713073.

Should I unzip the UTM or the BNG files?

For clients in the UK we recommend unzipping the BNG zip file which contains the mosaicked image converted to display in British National Grid projection. The UTM zip file contains the mosaicked image in Universal Transverse Mercator Zone 30N projection which is the coordinate system used by the GPS in the plane.

What is the difference between GeoTiff and ECW? Which should I use?

GeoTiff is the original format for the output images and ensures that no detail of the image is lost. This format is therefore recommended for analysis and printing purposes. However this does mean that the file size is considerably larger and it can take a while to draw on screen.

ECW is a compressed format which displays quickly but does lose some detail. In addition if the background is set to transparent some pixels will remain around the edges due to the compression. Nevertheless it is generally a lot faster and is therefore recommended for displaying on screen as a background layer.

When I open the layer in ArcGIS it says the projection is different to the map, what should I do?

This message is displayed when the projection of the layer does not match the existing data frame projection. Click on the 'Transformations...' button.

To display in a data frame set to British National Grid use the following settings:

• UTM: Convert from GCS_WGS_1984 into GCS_OSGB_1936 using OSGB_1936_To_WGS_1984_Petroleum
• BNG/ECW: Convert from ORDNANCE SURVEY 1936 into GCS_OSGB_1936 using <None>

Is it possible to remove the black area around the outside?

Yes, you can set black as transparent in the Layer Properties. This will hide all pixels for GeoTiff layers but not for ECW layers. Instructions are given below for the three most common GIS packages:

• ArcGIS

  Right-click on the layer and select Properties. Click on the Symbology tab and tick 'Display Background Value'. Leave the values as 0 and the drop-down as No Color.

• MapInfo

  Right-click on the layer and select Layer Properties. Tick Style Override and click on the button below. Tick 'Transparent'. Click 'Select color' and click on the black area. Click OK on both dialogs.

• QGIS

  Right-click on the layer and select Properties. Click on the Transparency tab and enter 0 in the 'No data value' box then click OK.

How can I display the tiles as a single layer in my GIS?

To display the tiles as a single layer you need to create a catalog or a seamless layer. Instructions are given below for the three most common GIS packages:

• ArcGIS

  In ArcCatalog, create a geodatabase then click on the Toolbox button. In the toolbox go to ArcToolbox > Data Management Tools > Raster > Raster Catalog > Create Raster Catalog.

  To create the catalog:  set the Output Location to your geodatabase and enter a Raster Catalog Name, then set the Coordinate System for the Raster and Geometry columns to British National Grid. Finally set the Raster Management Type and click OK.

  A Managed catalog imports the image data into the database, whereas an Unmanaged catalog will only store the path to the images.

  Browse to the geodatabase you created, right-click on the catalog and select Load > Load Data. Open all of the individual tiles, then click OK.

• MapInfo

  Open all of the individual files into a single map window. Go to Tools > Tool Manager. Next to Seamless Table Manager, tick Loaded then click OK. Go to Tools > Seamless Manager > New Seamless Table. In the Save dialog, enter a name for the new layer and click Save.

• QGIS

  A 'Virtual Raster' file was supplied with the data and can be opened as a raster layer. Set the drop-down on the Open dialog to [GDAL] Virtual Raster (*.vrt *.VRT) and browse to the file.

Compressed (zipped) Folders Error on Windows XP

The file is not actually corrupt. Windows XP cannot handle zip files over 2Gb. The solution is to either: use a computer running Windows Vista or Windows 7; or download zip software such as 7Zip (opens in a new window).