Introduction to rangefinders
Rangefinders are devices which, at a minimum, measure distance from a rangefinder, or observer, to a target. There are a variety of rangefinders available for different purposes, including rangefinders for measuring distances for construction, golf shots, speeding vehicles, and GIS data collection. Rangefinders are also known as laser rangefinders, distancemeters, and laser locators. In ArcPad, the term ‘rangefinder’ will be used for ease of communication.
In addition to different terms used for rangefinders there are also different terms used for the measurements made by rangefinders. Distance is also referred to as range, and can also be more accurately described as horizontal distance, slope distance, or vertical distance. Bearing is also referred to as azimuth or angle. Bearing can be relative to Magnetic north or True north. Inclination is also often referred to as pitch. In ArcPad, we will use the terms distance, bearing, and inclination.
Rangefinders are typically used for GIS data collection for the following situations:
- When mapping the location of an object which is inaccessible, either because it is difficult to get to or it is not safe to get to the object. For example, a tree on an island, or a manhole in the middle of a busy road.
- When mapping the location of an object where it is not possible to get a GPS signal, or a GPS position of sufficient accuracy. For example, under a large tree or in a narrow street or ‘urban canyon’.
Rangefinders designed for GIS data collection typically measure distance and inclination, while more expensive rangefinders also measure bearing. With ArcPad, you can use a rangefinder which only measures distance, or which measures a combination of distance plus inclination and/or bearing. You can also use a ‘rangefinder’ which only measures bearing—also known as a compass!
Rangefinders are used to take measurements that are then used to calculate the offset of the target relative to one or two known reference points. The offset is used to calculate the location, or coordinates, of the target based on the coordinates of the rangefinder or observer. Inclination is used to calculate the vertical distance, or elevation difference, between the observer and the target. The vertical distance can then be used to calculate the Z-coordinate of the target—based on the Z-coordinate of the observer’s reference point.
The easiest rangefinders to use for GIS data collection are those that measure distance and bearing. In this case only a single distance and bearing measurement is needed to compute the location of the target. Inclination is also needed if the Zcoordinate of the target is to be calculated. This is referred to as a distance-bearing offset measurement. However, rangefinders which measure both distance and bearing tend to be more expensive than rangefinders which only measure distance (and, optionally, inclination).
Rangefinders which only measure distance or only measure bearing can also be used to map the offset location of an object. In this case, two distance (or bearing) measurements are needed to compute the location of the target, from two known reference points to the target. This is referred to as a distance-distance (or bearing-bearing) offset measurement.
Although rangefinders report measurements to at least one decimal place it is important to understand that there is some amount of error inherent in these measurements. A number of factors contribute to this error, including the quality of the rangefinder, the distance to the target, stability of the rangefinder when taking the measurement, magnetic interference, charactersitics of the target, poor weather, and daytime solar conditions. Some rangefinders have options to minimize error, such as a poor weather setting to minimize the impact of rain, snow, smoke, or airborne dust particles. The best approach to minimizing errors, however, is to be aware of the above factors, and to select a rangefinder that is appropriate for your application needs.
The three most important factors for introducing error are:
- The quality of the rangefinder: The quality of the rangefinder is directly proportional to the cost of the rangefinder. More expensive rangefinders use better quality components and as a result provide more accurate measurements at greater distances. Less expensive rangefinders provide less accurate measurements, and are only suitable for measuring shorter distances.
- The distance to the target: The measurement error increases as the distance from the observer to the target increases. For example, a compass error of 1 degree for a bearing measurement results in a horizontal error of 1.75 feet at a distance of 100 feet, and 8.73 feet at a distance of 500 feet.
- The pointing stability: To achieve maximum accuracy, it is recommended that you use a rangepole, monopole, or tripod to stabilize the rangefinder—especially when measuring large distances. It can sometimes be difficult to target small objects at large distances when holding the rangefinder in your hand without any stabilizing support.
Working with rangefinders
Rangefinder measurements are used by ArcPad in the Point/ Vertex dialog box to calculate the coordinates of the target object. Rangefinders output distance and angle (bearing or inclination) measurements using different units. When ArcPad receives data from the rangefinder, it converts all distance measurements to meters, and all angle measurements to degrees. ArcPad then displays the distance measurements using the units of the current map projection, or the display units for Geographic projections. The rangefinder protocol and communications parameters are set in the Rangefinder Preferences dialog box.
When ArcPad receives data from the connected and activated rangefinder, a sound is played to notify the user. ArcPad also automatically opens and populates the Point/Vertex dialog box if the point, polyline, or polygon tools are active and the corresponding Offset Point, Linear Traverse, or Radial Traverse modes are enabled.
The 2 Point Offset page of the Point/Vertex dialog box is automatically displayed and populated if Reference points A and B have been specified. The first measurements from the rangefinder will be used to fill in the measurement fields for Reference point A. The next set of measurements received from the rangefinder will be used to fill in the measurement fields for Reference point B. At any time you can edit the measurements in the fields in the Point/Vertex dialog box. Tap OK when you are satisfied with the measurements and are ready to use them to create a point or vertex.