54 Crown et al.: Every Tree Counts: New York City Street Tree Inventory in TC2015, eliminated the option of “excellent” in order to leave the top condition category for park trees, which generally have more favorable grow- ing conditions; this served to reduce the number of categories citizen scientists had to learn. Tree Circumference to the nearest inch was collected in TC2015 rather than DBH, in order to eliminate confusion over which side of a diameter tape to use. These measurements were converted to DBH post-collection. Additionally, the shiſt to web- based data collection greatly reduced the number of variables collected by completely eliminating collection of metadata as described in Table 1. DATA QUALITY Spatial Extensive QA was conducted on tree locations by comparing the locations of the trees inventoried, as determined by linear referencing, and the loca- tion of trees on current, orthorectified aerial im- agery. If the tree locations were not within 3.05 m of the stems on the imagery, the entire survey was rejected. The overall survey rejection rate was low for both NYC Parks’s staff (2.2%) and citizen sci- entists (3.9%). These numbers do not take into account the ≈7% of data that was edited for mi- nor inconsistencies during the QA process. These overall numbers were calculated by pooling all surveys collected within each data collector group and dividing the total rejected by the total col- lected. Individual user rejection rates show varied performance between lower- and higher-level con- tributors. Users who contributed ≤10 surveys had an average rejection rate of 14.9%, whereas those who contributed ≥25 surveys had a rate of 3.9%. These numbers were calculated by averaging the individual user rejection rates within each category. Inventory Variables To measure volunteer accuracy for non-spatial vari- ables, NYC Parks’ staff resurveyed approximately 1% of volunteer-collected trees. Comparative results can be found in Table 2. Percentage agreement was calculated for all variables, and sensitivity (Sn), specificity (Sp), positive predictive value (PPV), and negative predictive value (NPV) were calcu- lated for all presence/absence variables. All of the calculated statistics are judged based on the follow- ing scale: high >85%, moderate = 70%–85%, low <70%. When calculating Sn, Sp, PPV, and NPV, staff data are considered to represent the actual state of the specimen, and volunteer data are treated as the test data. See Table 3 for clarification of Sn, Sp, PPV, Table 2. Table of results for comparisons between citizen-scientist data and data collected by expert staff, listed from highest to lowest % agreement. The number of times a presence/absence variable was noted as ‘Present’ by staff and citizen scientists is included to provide additional clarification for % Agreement results, which tend to be higher for those variables present in fewer cases, even in cases where Sn and PPV were low. n = 2872 for all variables except for DBH (n = 2913 due to the inclusion of dead trees) and Species (n = 2702 due to some specimens only being identified to genus level). Assessment variable Trunk Problem: Lightsy Root Problem: Metal Gratesy Trunk Problem: Rope/Wiresy DBH (2.54 cm tolerance) Root Problem: Othery Branch Problem: Othery Trunk Problem: Othery Branch Problem: Lights/Wiresy Tree Guards Genus Stewardshipx Species Tree Condition Root Problem: Stonesy Sidewalk Damage DBH (exact) Percentage agreementz 99.8 99.2 96.4 92.7 91.9 91.2 89.4 89.4 87.8 86.5 80.6 77.6 74.9 73.7 73.3 32.0 Sn (%) 80.0 66.7 25.0 -- 29.4 35.2 40.0 59.4 -- -- 79.9 -- -- 63.0 58.3 -- z Citizen-scientist percentage agreement with NYC Parks’ staff. y These assessment variables are sub-variables of Tree Problems. x Stewardship was converted to presence/absence for analysis because some signs of stewardship were ephemeral (for example, witnessing a steward watering or weeding a tree), and would likely not be present during both rounds of data collection, though the quantity category data will be used operationally. Sp (%) 99.9 99.3 97.3 -- 92.6 92.3 90.1 90.7 -- -- 80.9 -- -- 76.1 81.0 -- PPV(%) 50.0 23.1 10.6 -- 4.6 8.1 6.1 23.0 -- -- 70.3 -- -- 37.3 61.3 -- NPV (%) 100 99.9 99.0 -- 99.1 98.7 99.0 98.0 -- -- 87.7 -- -- 90.1 79.0 -- Staff presence (n) 5 9 36 -- 34 54 45 128 -- -- 1037 -- -- 529 931 -- Citizen scientist presence (n) 8 26 85 -- 219 236 297 330 -- -- 1159 -- -- 893 979 -- ©2018 International Society of Arboriculture
March 2018
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