A Background image for the map can be specified in this window. This will generally be a base map, but could be a logo, copyright message or simply a colour field. If a background is not defined, the map is drawn on the plain background of the page.

The image file is selected using the Select Background Image button. The dialog will initially select from files of types *.bmp, *.jpg, *.jpeg, *.gif and *.png. Other image types (e.g. TIFF) can also be opened if Quicktime is installed on the computer - an 'All Files' option is therefore available in the dialog, but the selected file may not open correctly.

Though by default, the map symbols will be drawn over the background, the user can choose to Treat image as a transparent overlay. Any transparent areas within the image (e.g in a PNG file) will be treated as transparent in the overlay, but white pixels will also be treated as transparent.  In order to show the plotted data symbols through the overlay, this is only really suitable (for example) for a line drawing or sketch map drawn on a white or transparent ground. In the preview, the transparent areas will the be shown as a pale blue.

If the Map Area is already defined, its outline is shown in the Preview as a green box - as shown above.

For most cases, the Background Image will be a base map.  Though the image must be rectangular and drawn on the same grid system as the data to be mapped, the image may not cover the same area as the map being drawn.   It is then necessary to Define the full grid area covered by the selected image (as shown in the above example).  The appropriate portion of the image will be then be used (and if necessary extended) to match the Map Area, even if the Map Area changes later.

The minimum and maximum Eastings and Northings values for the background image can be defined by the user and the image will be effectively stretched to these coordinates before being cropped or extended to the Map Area.  Note that if the Eastings 'width' and Northings 'height' do not match the width/height ratio of the supplied image, the image will be stretched and may be distorted before it is matched to the map area.

Alternatively, if the project uses OS NGR, the image area can be defined by entering the OS NGR strings for any two opposite corners of the image.

If the Background image already matches the required Map Area, or if it is a logo or colour field there are several 'Fit to ...' options available as alternatives.

Given the above example of a 'tall' Map Area and a 'square' background shown by the cross-hatch pattern, the background can be 'fitted' to the Map Area either in the centre, at the top (in fact to the top/left), at the bottom (bottom/right), or it can be stretched to fill the whole Map Area.  The 'Fit' options may be appropriate if the background were just a logo, the 'stretch' possibly if the background was just a colour field.

If the background image is no longer required, it can be removed using the Remove Background button.

TIP - For UK data on the NGR grid, outline and administrative maps for the UK are available free on the Ordnance Survey Web Site, as are a range of OpenData maps at scales from 1:1,000,000 to 1:10,000.  These free maps include the 1:250,000 'Quarter Inch' maps of the country, and StreetView maps, nominally at 1:10,000, but not the Landranger and Explorer (1:50,000 and 1:25,000), though the OS 'VectorMap' is at the nominal scale of 1:25,000.  Note when ordering such maps that you will need the 'Raster' version for use with DatMapR.

If the data is held as Latitude/Longitude, the appropriate World Map required as a background is the Plate Carrée or Equirectangular projection (see Wikipedia for examples).