Tip on Map Backgrounds

A frequent question is how to get good background images for use with ArcPad on the handheld PDA in the field.

For many people one good option can be using aerial photo image TIFFs, in particular already georeferenced USGS Digial Orthophoto Quarter Quads (DOQQ) or NAIP (National Agricultural Image Program) quarter quads from the US Department of Agriculture. These are available with 1 or 2 meter per pixel resolution (depending on location and time), and they are often available for free.

The quarter quad format covers a quarter of a standard USGS 7.5' grid (originally defined by the famous USGS topo maps). That makes it roughly 3 x 4 miles (depending on latitude), which is a reasonable size of building block. In some cases one quarter quad could cover the entire target area, or in other cases coverage might take a good number of quarter quads. One good thing about these quads it that the quarter quad files which are "off the screen" do not take up much of the memory or processing power of the handheld PDA, so it can be more responsive as you zoom in.

The USGS DOQQ images are typically about 1 meter resolution in black and white, and may be fairly old, depending on your location. The USDA has tried to create NAIP images every few years; some are in color, some are in black and white; some have 1 meter resolution, some have 2 meter. You will just have to see what's available for your area. We will mention sources below.

We have found that uncompressed TIFF files (.tif) work well on our Recon. You can also use compressed image formats like JPEG (.jpg) or Mr Sid (.sid) or ECW (.ecw); these are considerably smaller but on our Recon at least they are also much slower to redraw on the screen. TIFFs work well because ArcPad is optimized to fetch just the part of the file it needs to display (whereas it has to uncompress a lot more data from the uncompressed files, then clip to the viewable portion). The difference in user experience can be substantial - it's a real pain if ever twitch of the screen (move, zoom, or even just add or remove a toolbar) takes 20 seconds to redraw.

The downside is that the uncompressed format takes more space. A typical USGS DOQQ (black and white, 1 m res) is about 46 Megabytes, while a 1 m res color NAIP quarter quad is around 135 Mbytes. These may be way too big to hold much in your handheld PDA's internal memory. Of course, you can delete yesterday's images and load todays before going out, but that's potentially time consuming.

Our current solution is to use a removable memory card for our handheld PDA/GPS unit, a Trimble Recon. The Recon uses Compact Flash format cards (check your device to see what format it uses; most accept some format). With a 4 GByte compact flash card, we can put DOQQ and NAIP images of the whole Sonoma Valley on a card. For a given project you will still load only the ones which are relevant as layers in ArcPad, but that's relatively quick (and easy to change in the field if your plans change).

One big benefit besides just plain holding so much data, is that these maps stay in the card even if your battery dies in the handheld PDA. Sometimes internal memory is erased if the battery runs out (sometimes not).

Be sure to check or test the maximum size memory card your handheld allows - sometimes there are internal limits to how large it can use, even if the card format allows larger cards to be made. 4 Gigabytes seems to work for our Recon, but we don't know if 8 or 16 GBytes would; others might have smaller limits.

If your memory cards are easily inserted and removed, you could even buy several for different sub projects, and swap them.

Moving gigabytes of data onto the cards would take many hours using the Recon's USB cable (newer units could be much faster). However it's easy and very fast to load the cards directly by plugging them into a card reader attached to the computer (via USB or sometimes using slots on the front panel of a desktop or the side of a laptop). We create a folder called "rasters", then keep images in subfolders within that (DOQQ and NAIP in parrticular).

Be sure you copy over the other files with each tiff as well, in particular the .tfw world file and perhaps an XML metadata file; these help ArcPad know where the image should be mapped to the earth's surface.

These two image formats are produced by the US federal government and are not copyrighted, so it's possible for various organizations to distribute them for free - if they have a budget to support the neccessary web servers, for example. In California, quarter quads in both formats for the whole state are available from CaSIL (California Spatial Information Library). You can find it with a web search (if you get old links, look for another mirror - they keep moving it. Other states or universities often provide access to similar data; the library website formats vary and change however.

Which quarter quads should you get? This tip is getting kind of long so that won't e covered in detail - look for information within the spatial library you use. Often they are organized by one degree squares (38120 would be 38N 122W), and then within that by either a letter / number code for the 8x8 grid of topo quads in each degree, or by quad name. Then the quarter quads are named with NE,NW,SE,SW for the four corners of a quad. There may be an index, as a shapefile or interactive, to help you find the ones you want.

Much of this applies to other possible background layers as well - other aerial imagery, or street layers, or generated terrain. In particular the idea of segmenting it into independent, chunks like the quarter quads, using TIFF format for speed, and adding enough storage to support multiple uncompressed maps.

One final note - sometimes NAIP data is delivered in county size mosaics; these are typically several gigabytes in a compressed format like MrSID, and will tend to perform very badly even if your memory card supports them (at least in our Recon; you might have a much faster processor and more RAM, so feel free to give it a try - in the office first!). You may need to have a GIS person segment these into quarter quad like chunks if you cannot find Quarter Quad formats directly.

The Geoweed Team