On field survey methods using handheld GPS

The previous post dealt with the various software packages we use for desktop mapping, ArcView, ArcCatalog etc. In this post we will look at the software we use to complete mapping tasks in the field.

ArcPad is a mobile version of ArcView and can perform many of the same functions as its desktop counterpart, but is designed for handheld devices used in the field. The main benefit in using ArcPad is the ability to combine map data with a GPS receiver on the mobile device. The GPS receiver, through various conversions, communicates with ArcPad to plot the location of the Trimble (and thus the person holding it) on the screen, relating it to the relevant map data.

After a few minutes to allow the GPS receiver to pick up a sufficient number of satellites, a crosshair appears on the screen indicating our location. As we work, the receiver will track our movements in real time and ArcPad will update our geographic position on the screen map, including point locations and shape files we have added. Thus, we can use the combination of the GPS position and the locations of the historic lime mines (stored in the handheld’s memory in Vector format) to guide us to the location of a specific mine, within an error range of a kilometre or so.


Image of ArcPad using GPS to plot a current position (crosshairs) on the Trimble Handheld. The points represent either historic mines or sites added to our database, depending on what colour they are.

Not only can we use existing files in the field, ArcPad also allows us to edit blank shapefiles and add data for new locations in the field, along with text-based attributes to describe relevant features of each location. This is another great advantage to mobile GIS. As explained in the previous post we use ArcCatalog to create blank shapefiles. Once these are loaded onto the handheld and opened in ArcPad we can use the GPS receiver to record our location and store it within the shapefile. This is how we add new locations to our database and record a more accurate location for any historic lime mines we find.

A major downside to using handheld platforms is that they do not have particularly quick processors. As such, using large raster data sets, like topographical maps, is tricky. It is possible to convert raster data to a size that ArcPad can handle, but this has limitations on the amount of data it can convert. Consequently only a very small section of a topographical map can be converted for use on the handheld. For survey work over large regions, this is not really useful.

Given the large scale of the project it has not been feasible, or deemed necessary, to perform this conversion over and over again for each of the regions we have visited. Rather we have mainly focused on using the digitised vector data to represent point locations and geological units extracted from the mining and geological maps for use on the handheld. This takes quite a bit of time to prepare but has provided invaluable information to us in the field. Once we have finished surveying for the day we download the shapefiles with the new location data on them onto our tablet PC and add them as new layers to ArcView. We can now view the points along with the topographical map, enabling us to view the new information in a wider context.

To summarise, ArcPad allows us to:

  • Know our exact location through the GPS receiver
  • View pre-existing Vector shapefiles like our historic mine location database and geological formations
  • Add new data blank Vector shapefiles with points or polygons captured using the GPS receiver
  • Upload the new information to our tablet PC viewing it in a wider context and with other relevant data like topographic maps

Without ArcPad and the GPS function of our handhelds relocating historic lime mines and plotting the accurate location of new sites for our database would be incredibly difficult. Using ArcPad and ArcGIS for mapping also has the added advantage of being able to connect directly to our project database in Microsoft Access and update our field data on daily basis. The next post on our survey procedures will deal with the database software and how we use both desktop and handheld-based software to record the sites we visit, and how all the components work together.

– A Reid

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Geology maps: where is that dolomite?

Apart from the two historic mining maps we have acquired, a substantial amount of our information has come from 1:250,000 maps of the bedrock geology. These provide the geological characteristics, and the location, of the underlying rocks in any area of South Africa. Because the types of caves we are looking for form in dolomite, locating areas where this rock is present is one of the main factors in determining our survey strategy.

The geology maps have been treated in the same way as the historic mining maps, utilising only the information relevant to the project and converting that information into a usable digital format. However, unlike the mining maps, on which the digital information is characterised by a point, the information we require from the geology maps relates to the region (including the size and shape) occupied by a lithological unit of bedrock.

Geology base map. An example section of a 1:250000 geological map showing areas of dolomite in different shades of blue (map published byDepartment of Mineral And Energy Affairs, Republic of South Africa 1981).

 

In order to convert this map information from Raster to Vector formats, we trace around the edges of the bedrock formations we are interested in, save them as a polygon (instead of a point), and then add the lithological information as an attribute of the feature. Not only does this kind of digitisation help in establishing areas to investigate but it also has a number of other advantages.

Geology map with polygons.  In this figure, the blue areas denoting a dolomitic lithological unit have been overlain by a traced polygon using ArcGIS and are now represented by different colours to indicate different attributes the rock unit might have, such as including interbedded chert, quartzite, or shale (map published byDepartment of Mineral And Energy Affairs, Republic of South Africa 1981).

The first advantage is the reduction in size of the file from a large scanned map which may be as much as 390 Megabytes per file to a few Kilobytes of data. This is helpful not only when trying to use the data on a handheld computer but also for storage on the device, since we can fit many more polygons than scanned maps on a 4 Gigabyte memory card. The second advantage of digitising in this way is that we can select the only data that are  relevant to the project and its aims. We can ignore all the other geological formations which do not hold the appropriate characteristics and only digitise those that we need. This again saves space on the memory cards but also saves time, both in the tracing of the geology and in locating regions appropriate to the project in the field.

Geology polygons. This figure shows how the digitisation of the geological formation looks when the area is loaded onto the handheld computers. The pink and orange colours represent the (dolomite) geology and the green background represents the Vector outline of the main provinces of South Africa.

The “Identify” function on the GIS software (ArcPad) we use on the hand-held computers can be used to call up the attributes of any of the Vector layers  once defined. The GPS function on the handheld also enables us to determine our current field position relative to the geological formations (polygons) and to the mines (points). This helps us to avoid spending a lot of time driving around searching in the field!

This is the final step in the digitisation process to prepare the maps for the field. The next blog in this series will deal with some of the hardware and software we are using in more detail.

– A Reid