On-screen digitizing is an interactive process in which a map is created using previously digitized of scanned information. This method of geocoding is commonly called "heads-up" digitizing because the attention of the user is focused up on the screen, and not on a digitizing tablet. This technique may be used to trace features from a scanned map or image to create new layers or themes. On-screen digitizing may also be employed in an editing session where there is enough information on the screen to accurately add new features without a reference image or map.
The process of on-screen digitizing is similar to conventional digitizing. Rather than using a digitizer and a cursor, the user creates the map layer up on the screen with the mouse and typically with referenced information as a background.
The following two examples are scenarios that illustrate the use of on-screen digitizing.
A tax mapping facility is charged with the responsibility of updating the planimetric property maps that the county Assessor uses to compute taxes. Although the county is committed to using a GIS to store the property information and to compute land areas, the accuracy of the existing maps is less than ideal. Rather than assume the nearly impossible task of making the entire GIS fit the recent updating information, the cadastral mapper fits the updated property lines to the existing data.
One lot in question needs to be split off into two separate lots reflecting a recent subdivision and sale of property. The legal description includes information dealing with a nearby road intersection, a section line, and the edge of an irrigation canal. The GIS dataset has layers that comprise road networks, township and range information and public works and canal information. These layers are consistent with the cadastral plan of the Assessor's property layer.
In the GIS editing process, the roads, canals and section lines are drawn as a background in separate colors. Armed with the proper information, the new line that reflects the split of property may be added using the proper reference of the background layers. It is important to realize that the background layers are not being edited themselves, they are only being referenced.
This process was necessary because the new data may have been more accurately digitized with other map sources but it would not have "fit" the less than ideal property map that is already in existence.
This example uses a scanned image as the background reference in the heads-up digitizing approach.
A groundwater hydrologist is interested in modeling the impact of potential nitrate leaching in a particular study area. Two structures have been identified as being possible sources of pollution. The greater threat appears to be from animal confinement operations and to a lesser extent, a small percentage of septic tanks for rural homes could possibly be leaking.
None of the paper map sources has the detailed information necessary to readily digitize this information from. Fortunately, the hydrologist has obtained recent aerial photographs at a scale appropriate for discerning houses and feedlots.
To create the two layers identifying feedlots and houses, the images are scanned and used as a reference background in the editing process. The septic tanks are assumed to be near enough to the house for modeling purposes and a point is created for each house in that layer. Using the background image as a reference, the animal confinement locations are digitized by tracing the boundaries of the feed lots and any sludge lagoons that may be on the premises.
In this example, the accuracy of the digitized septic tank and feedlot locations is only as good as the resolution and accuracy of the scanned images.
The following list describes the expected skills which students should master for each level of training, i.e. Awareness/Competency/Mastery.
The expected learning goals of this section are to achieve a general understanding of process of on-screen digitizing and its potential uses. The student should also posses a working knowledge of traditional digitizing (with a digitizing tablet) methodologies.
The learning goals of this section are to develop the ability to perform specific on-screen digitizing operations. The student should be able to apply traditional digitizing techniques in order to update existing layers or create new ones on-screen using other layers in the background of an editing process.
The learning goals of this section are to be able to make all appropriate decisions in the digitizing process from beginning to end of a particular project. The student is able to use on-screen digitizing with images scanned from maps, photographs, or satellite data if suitable. The student should also be aware of accuracy issues dealing with these sources of data.
Recommended:
Complementary:
Learning Objectives:
Vocabulary:
Topics
Digitizing on screen as well as traditional digitizing vary widely amongst software/hardware systems.Learning Objectives:
Tasks:
This process uses traditional editing techniques, the only difference is that a digitizer is not the input device. Map registration is not necessary, it is assumed that the updated features will be placed in the proper location using visual cues from the reference layers.
This process is exactly the same as updating a layer.The registration of the coverage needs to be addressed.Learning Objectives:
On-Screen Digitizing Tasks
All GIS systems do not have this functionality. Data can be exported to many spreadsheet software systems if necessary.
Suggested:
The ESRI Arcscan site and a white paper dedicated to automated digitizing techniques. An example of using heads-up digitizing to delineate contaminate areas in wellhead protection by Christopher Barnett, Yan Zhou, Steven Vance and Christopher Fulcher from the Center for Agricultural, Resource and Environmental Systems at University of Missouri - Columbia. An example of medical facility placement that utilizes heads-up digitizing. An example of heads-up digitizing in identifying locations of the Mojave Desert Tortoise. Resources
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Currently maintained by Steve Palladino
Created: May 14, 1997. Last updated: October 5, 1998.
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