Estimated abundance in covered region

[Ariek Norford]

The estimated abundance in covered region is smaller (29.83) than in the total region (60) even though the covered area (3,553,200) is larger than the area (1,500,000). Is this an error others have encountered? I am wondering if I am misinterpreting the results. 

When I multiply the density (4e-5) by the area, it gives me the correct value for the total region, but not for the covered region. 

[Eric Rexstad]

Ariek

Good practice for you to be checking the summary statistics in your output to look for possible problems.

But I'm not quite sure what you are asking, so I'll walk through the calculations. I am speculating this might be point transect data and show calculations for such:

• it is possible for the covered region to be larger than the size of the study area (that is often confusing for folks)
• covered area for each point is (pi * truncation distance); but there are multiple points, so include a multiplier for number of points (k). However, sometimes investigators visit point stations multiple times, which must be incorporated into the covered area calculation: (visits) * (k) * (pi * truncation distance)
• how is abundance in covered area computed?
• that is simply the number of detections divided by probability of detection in the covered area:
• n_detections / P_a
• abundance in the study area is calculated as estimated density multiplied by size of study area (as you noted)

Does this resolve your question?

---

From: 'Ariek Norford' via distance-sampling <distance...@googlegroups.com>
Sent: 20 July 2025 04:09
To: distance-sampling <distance...@googlegroups.com>
Subject: [distance-sampling] Estimated abundance in covered region

 

The estimated abundance in covered region is smaller (29.83) than in the total region (60) even though the covered area (3,553,200) is larger than the area (1,500,000). Is this an error others have encountered? I am wondering if I am misinterpreting the results. 

When I multiply the density (4e-5) by the area, it gives me the correct value for the total region, but not for the covered region. 

--
You received this message because you are subscribed to the Google Groups "distance-sampling" group.
To unsubscribe from this group and stop receiving emails from it, send an email to distance-sampl...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/distance-sampling/ebe42e8c-00c6-4076-8d4b-b802400f5fa4n%40googlegroups.com.

[Ariek Norford]

Thank you for your response! I understand now how the abundance in the covered area is calculated. However, I still don’t understand why it would be so much less than the estimate for the total region (which is already extremely high compared to literature values). I couldn’t find a message previously about this on this google group (though it may exist). Are there characteristics of the data that may create such a scenario?

For more context, I walked line transects. However, I walked them multiple times. I had 2 km of transects in each region that I walked 3-4 times. So for the distance, I entered that as 6-8 km depending.  Maybe this is what caused the error? 

[Eric Rexstad]

Ariek

Multiple visits to the transects is reflected in effort (effort = actual transect length x number of visits) exactly as you have done. This can result in the size of the covered area being larger than the physical size of the study area (as in your case).

Abundance in the covered region (~30 in your case) divided by size of the covered region (3553200 for your survey) results in an estimate of density in the covered region. The covered region is assumed to be representative of the entire study area, hence the computed density estimate (30/3553200=8x10-6 per m?) is assumed to be the density of the entire study area.

Multiplying the estimated density by the size of the study area results in an estimate of abundance for the study area. However, I cannot duplicate your study area abundance estimate of 60 from the other information you provided. If you are working with the Distance R package, you could send off-list your output or better yet, your code and input data frame.

None of these computations are dependent on characteristics of the data. I think you might be placing too much emphasis upon the estimated abundance in the covered area; it is simply an intermediate step in the computation of density and/or abundance in the study area, which is the purpose of the survey.

---

From: 'Ariek Norford' via distance-sampling <distance...@googlegroups.com>

Sent: 21 July 2025 19:25
To: distance-sampling <distance...@googlegroups.com>
Subject: Re: [distance-sampling] Estimated abundance in covered region

 

To view this discussion visit https://groups.google.com/d/msgid/distance-sampling/3ec88f84-80d7-4b7e-ae70-a68e72b9920fn%40googlegroups.com.