Fisheries and Land Resources

Forestry and Agrifoods

Implication of Detailed Photo Interpretation

Several years ago it became evident that photo interpretation in the second inventory cycle carried out in the 90's is significantly more detailed than in the first cycle carried out in the 70's and 80's. To estimate the affect of this on production levels in the Mapping Section a small study was conducted to compare the 1977-1978 inventory with the 1993-1994 inventory in District 16. The study demonstrated that more detailed interpretation has significantly increased the costs (time and money) to create digital Forest Inventory Maps.


A comparison was made between the 1977-1978 inventory and the 1993-1994 inventory for map sheet 50. Three things were compared:

  • Arc Length - This is the total length, in kilometers of all lines including roads, rivers, lake perimeters and stand boundaries. There was a 43% increase in this value.
  • Polygons - This the total number of polygons including lakes, non-forested and forested areas. There was a 103% increase (approximately double) in the total number of polygons.
  • Stands - This is the total number of forested stands. There was a 205% increase (approximately triple) in this value.

These results are depicted in the following table

Year of Photography Arc Length Polygons Forest Stands
1977-1978 5730 4920 1724
1993-1994 8192 10007 5240
Percent Increase 43 103 205

Clearly there has been a significant increase in the level of detail on FI maps created today as compared to twenty years ago, and this has the following implications:

  • Increased time to interpret photographs;
  • At least 43% more time to digitize lines;
  • At least twice as long to digitize polygon identifiers;
  • At least three times as long to keypunch stand attributes data;
  • Polygons are on average half as large and therefore maps are more complex. This makes label placement much more difficult and significantly increases the quality control effort.
  • Larger files and increased processing time.

There are several possible reasons why interpretation has become more detailed.

  • Breakup of stands due to increased disturbance/mortality/insect damage
  • More skilled photo interpreters
  • Demand from users for more detail
  • Relaxation of guidelines to meet demands of users
  • FI maps, intended for planning purposes, have become operational maps

The increased detail is happening at a time when there are more demands to cut costs and improve production times. We must ask ourselves whether we need such detailed maps to effectively carry out ecosystem management? If indeed the detail is required, we must be prepared to commit resources to deal with the extra work effort.


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