What to do when GPS signals are blocked by heavy canopy?

“I am currently using the program to plot a property line for our vineyard. One of the corners of the property is tucked deep within some woods where satellite reception is poor. Therefore, the program is not allowing me to record the point. Is there any way to work around this? Perhaps increase the rms limit to within several feet so as to allow for data collection? Thanks for the help.”

This is why iCMTGIS PRO provides the Traverse function to deal with a situation where GPS signals are blocked by heavy canopy. How far is this corner away from a point at which you have good satellite signal reception? What you could do is to record a good point at the periphery of the wooded area then measure the distance and bearing from this point to that corner. Then you will use the Traverse function under GPS – Collect data to specify the GPS point as the starting point and enter the distance and azimuth or bearing information to create the corner point.

You could try and see if you could record the point by relaxing the RMS limit and the DOPS limit, but the position accuracy will be compromised.

If the accuracy requirement of this point is not strict, and you are able to identify the location of this corner on a background map, you could digitize it by using the Add Point Manually function. Zoom in to better place the point.

If you have a deed document including the property description (distances and directions around the perimeter), you could use the Deed Calls function to enter such information in iCMTGIS PRO to draw all or part of the property boundary as a reference for digitizing the corner point.

After you have created the corner point, you can add it to the remainder of the GPS’d property boundary Area by using the Insert Node function.

How to create new point, line and polygon features with associated attribute fields and values?

There are a few ways of adding Points, Lines and Polygon Features to a map job.

1) Digitization

You will first need to create a new Topic layer on which to store the Features. Specify the correct topic type (Point, Line or Area) for the New Topic. Also, Farming GPS GIS, Forester GPS GIS and Utility Data Collection will let you add a Topic from the Feature List associated with the map job.

The Tools menu provides the Add Point manually, Add Line manually and Add Area manually functions. When a Point Topic is active, you can digitize Point Features. When a Line Topic is active, you can digitize Line Features. When an Area Topic is active, you can digitize Area Features.

Use the Edit Topic function to add Attribute fields to the Topic.

After digitizing the Features, you can turn on Sheet View (the right-most icon at the top) and enter the values. Farming GPS GIS, Forester GPS GIS and Utility Data Collection provide the ability to select pre-defined attribute values from within the Sheet View.

2) GPS Data Collection

The iCMTGIS II, iGPSGIS II and Utility Data Collection software apps will let you map points, lines and areas using GPS. When you collect GPS data, you can record the attribute values at the same time.

If you have a Feature List defined and associated with a job, during GPS data collection you can simply select one of the Values instead of having to enter the data by hand. The Feature List function is listed under GPS. It lets you define the Feature Topic Layers, the Attributes and the Values for the Attributes as well as the GPS data collection parameters.

Select Main Menu\Job\Current Job to get to the Job Setup screen. Here is where you can select the Feature List to be associated with the active Job. The iCMTGIS II, iGPSGIS II and Utility Data Collection software provide a Feature List file named “park.fbr” as an example.

The GPS data collection function is provided under GPS. Please see the built-in Help document for the procedure for recording various types of points, lines and areas. This document is available in PDF format upon request.

3) For Point Features, there are also the Traverse method under data collection and the Add Point by Coordinates function that you can use to create new points.

What to do when GPS is not available

The TRAVERSE function in iCMTGIS II and iGPSGIS II is a tool that lets you continue mapping multiple Point Features when a good GPS signal is not available.

To use the Traverse function you will first establish a reference point. This could be the last GPS point logged, or any Point Feature in the job. The new points will be created based on slope distance, slope and azimuth information for the imaginary straight line going from the reference point to the new location.

You could use the Side mode to do multiple “side shots” from the same reference point. For example, after logging a GPS point outside of a cave, you could use that as the reference point and take distance and angle measurements to the artifacts you’ve discovered inside the cave. Enter this information into the Travers Side data fields and the locations of the artifacts will be recorded in your job file.

Or, you could use the Traverse mode  to traverse to a new point that will then be used as the reference point for the next traverse point. This method can be used for storing vertices for lines or area boundaries.

What to do when GPS signals are not available?

GPS mapping depends on your receiver’s ability to receive the signals sent out by the orbiting satellites and to compute its position. When working under heavy canopy or inside buildings, the GPS signals are considerably weakened or blocked altogether. What’s a person to do under those conditions?

As mentioned in our blogpost dated 7/17/11, you  could join a Wi-Fi network and use the Location Services on your iOS device to obtain position information, which is not as accurate as that obtained from a GPS device.

If you are running the iCMTGIS app, you could switch to the Traverse mode and record angles and distances that reference back to the last known position. So, suppose you would like to record the location of a number of artifacts that you’ve found inside a large cavern. What you could do is to first log a good GPS point near the entrance to the cave then use that point as the first reference point for the Traverse measurements inside the cave.