SURFACE WATER CHANGES IN PORTLAND OREGON 1880 – 2004
The surface water’s of Portland, Oregon is a physical system with a distinct history. Any assessment of a past condition is complicated by the fact that past conditions have been influenced in ways not detectable with map evidence alone. Spatial data obtained from map sources demonstrate quantifiable change to surface water features. The example of snapshot data presented here evaluates the snapshot model and its ability to quantify change. Combined with background information, this study documents surface water changes for a 120+ year period in Portland, Oregon using 11 snapshots.
Because the temporal information presented in map sources is limited to the state of a particular water body based on evidence contained in the source date. This study models snapshots in a temporal order and because there is no direct evidence that allows us to say that a particular event caused a particular spatial effect. Included are information not directly related to the source data, but which serve the function of linking snapshots to ongoing temporal events that produce change. For example, a short duration flood event does not get recorded even though the effects may be present on the snapshot. The flood event wouldn't be recorded as such even if it appeared on the source map because the snapshot database cannot differentiate it from other surface water.
Snapshot data increase our knowledge of the change to surface water over time by indicating the past location and extent of surface water compared to the present or other periods in the past. These snapshot data also allow queries to determine a time interval that defines a cartographic state. Limitations to the snapshot model are a result of the way time is stored in the database. The temporal content of the snapshot database is limited to the date the source map evidence was collected.
One limitation that affects the temporal information content of snapshot data involves the use of valid time. In the snapshot database the exact date of the final disappearance of Ramsey Lake does not appear because it was not recorded within the source data. Examples include snapshots overlain to demonstrate the reduction in surface area of Ramsey Lake between 1880 and 1961. The lake appears to get smaller between 1880 and 1961 until it does not appear at all on the 1977 snapshot. The snapshot's explain the behavior of the surface water as changes in size and shape until it finally disappears after 1961 and before 1977. The final disappearance date of Ramsey Lake according to these snapshot data is between 1961 and 1977. There are two sources of temporal information to model temporal relationships between snapshots: the independent time reference that links each snapshot with a source date and the temporal link to an event database. Snapshots are an effective way of representing "time slice" data for themes such as surface water in Portland, Oregon because temporal joining provides additional information that bounds each snapshot. The temporally joined information gives additional clues about events that did not appear in the source data. Combining source date information from mapping with temporally joined information aids in the interpretation of historic events that are responsible for change.