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Montana Spatial Data Management Technical Note

Subjects:

  • Tracking soil polygon history in digital spatial data development
  • Caution on removing common lines

Tracking Soil Polygon History in Digital Spatial Data Development

This issue applies to both initial mapping and update/maintenance work in digital soil survey map activities.

  1. The goal in modern soil survey is to ensure to the level possible, using current technology, that correlation is reversible.
  2. Every map change does not need to be tracked, but significant label edits and polygon splits that have potential historic value need to be tracked in the working official survey spatial data (currently an ArcGIS personal geodatabase).
    • Tracking can be accomplished, and valuable historic information preserved, by simply adding a column(s) to the ST###_a feature class (MT612_a for example).
    • This column would be named MUSYM_MM_YY (e.g., MUSYM_8_05 for August 2005). Character type: text, field length set as appropriate.
    • The current MUSYM column data would be calculated into this new field.
    • This new column would not be edited, but it would become an integral part of the historic tracking of a polygon's label.
  3. Over the life of an activity (initial mapping/maintenance), more than one snapshot of the current polygon's label might be necessary.
  4. The number of historic columns used is entirely dependent upon the number of times a specific polygon's label is modified or the polygon is split.
    • Determination is based on the lowest common denominator rule, meaning if a small number of polygons requires more than one historic MUSYM column, then it is added to feature class.

Very important! You still need to make backups/archives of your data on a regular basis. This process does not result in perfect absolute change tracking of your spatial dataset. You still need to make periodic snapshots of your data for future comparison/security purposes. Merging two polygons together for any reason erases/eliminates the historic record for all but one of the polygons involved in the merge. Subsequently, removing common soil lines from two or more polygons that have valuable historic records associated with them in the form of the MYSYM_MM_YY column causes this fragment of history to be lost.

The historic or tracking MUSYM column naming convention is revised from earlier recommendations. This current approach is to imbed as much reference information as possible. Inclusion of the month and year in the historic column name tracks the start and end months that a set of polygon configurations was in force.

Caution on Removing Common Lines

This issue is relevant in the context of the issue above, tracking historic polygon records.

All offices have been provided with an ArcGIS model to facilitate the identification and resolution of common line “errors” in your working copy of the survey’s personal geodatabase. The primary focus is on the ST###_a feature class (soil polygon layer).

Very important! Merging two polygons together for any reason erases/eliminates the historic record for all but one of the polygons involved in the merge. Subsequently, removing common soil lines from two or more polygons that have valuable historic records associated with them in the form of the MYSYM_MM_YY column causes this history to be lost. We need to track soil labels at the polygon level to ensure two-way correlations.

EXAMPLE:

  1. A symbol, 11A, is made additional to an existing unit 211A.
  2. The MUSYM_MM_YY will not be edited but will retain the additional symbol label, 11A.
  3. The MUSYM (current management column for soil polygon label) would be edited to reflect the label that the polygon was correlated to, 211A.
  4. This process accommodates split correlations, where one old symbol may go to more than one new symbol, which could result in a common soil line in the context of the MUSYM. Now we may have two 211A units that adjoin.
  5. We do not want to merge the soil polygons based on this new common soil line (MUSYM context of 211A) as it would result in the loss of the former label, 11A, for the polygon that just was correlated.

We do merge polygons to eliminate common soil lines, and we do merge polygons in general. The caution is that when performing quality control for common soil lines at the Soil Survey Office level we only remove common soil lines (merge offending polygons) when there is no relevant historic polygon information to be lost.

Soil Survey Offices are to run the Common Soil Line checks and validate resulting common lines. Common line checks can be done on any of the MUSYM columns. The resulting list of common lines need to be investigated and validated for historic value, merging only those that do not impact the historic tracking of polygon labels.

When the data is delivered to the State Office for processing to the Soil Data Warehouse/Soil Data Mart, the staff there will melt any common lines from the dataset, based on the current MUSYM polygon label record. This processing will have no affect of the “official” working database that still resides in the Soil Survey Office. This cleaned-up version is what will become available to the Field Office and general public through the Soil Data Mart/Web Soil Survey.

Contacts: Montana State Soils Staff, either SDQS or the Assistant State Soil Scientist, for further clarification or instructions.

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Last Modified: 12/10/2007