I am using an API to pull in some Web Mapping Service Layer data to my script. The API query has a max count limit of 1000 on returned features per request. There are 2 attributes I am trying to use in my query 1) A DFIRM_ID field that has the same value for multiple features (its not unique but there are many DFIRM_ID values) and 2) The OBJECTID -this is a totally unique value. Each feature has its own OBJECTID. My goal is to get the total number of OBJECTID's for a given DFIRM_ID (or the total number of features). So, I want to use Output Statistics in my query. Which also requires 'Group By Fields'. According to the ArcGIS REST API documentation, I believe I should have ``. But the outputStatistics part seems confusing (as to how to put it in my query. The documentation says the following syntax (or something like it) is to be used:

"statisticType": "<count | sum | min | max | avg | stddev | var>",
"onStatisticField": "DFIRM_ID", 
"outStatisticFieldName": "DFIRM_COUNT"

I am just a little confused. How would I use that in my params variable below when plugging it into my code?

params = {'f': 'json', 'where': 'DFIRM_ID like \'13245%\'+'and OBJECTID>='+str(minCount)+'and OBJECTID<='+str(maxCount), 'geometryType': 'esriGeometryPolygon', 'spatialRel': 'esriSpatialRelIntersects','outFields': '*', 'returnGeometry': 'false'}
r = requests.get('https://hazards.fema.gov/gis/nfhl/rest/services/CSLF/Prelim_CSLF/MapServer/3/query', params)
cslfJson = r.json()
  • 1
    I think something is missing in I believe I should have ``, right? – Berend Sep 28 '18 at 7:19
  • Yep. Sorry, it got removed somehow. – gwydion93 Sep 28 '18 at 17:12

The statistics you need are the count of OBJECTID for each DFIRM_ID, so your onStatisticField should be OBJECTID instead of DFIRM_ID. The groupBy parameter should be set to DFIRM_ID:

    "statisticType": "count",
    "onStatisticField": "OBJECTID", 
    "outStatisticFieldName": "Count"

Which would make your params variable something like this

params = { ....  'groupByFieldsForStatistics': 'DFIRM_ID', 'outStatistics': '[{"statisticType": "count", "onStatisticField": "OBJECTID", "outStatisticFieldName": "Count"}]' }

An easy way to find out what the URL should look like is to use the HTML interface of the rest endpoint: https://hazards.fema.gov/gis/nfhl/rest/services/CSLF/Prelim_CSLF/MapServer/3/query?f=html

This results in URL (empty parameters removed): https://hazards.fema.gov/gis/nfhl/rest/services/CSLF/Prelim_CSLF/MapServer/3/query?where=1%3D1&groupByFieldsForStatistics=DFIRM_ID&outStatistics=%5B%7B%22statisticType%22%3A%22count%22%2C%22onStatisticField%22%3A%22OBJECTID%22%2C%22outStatisticFieldName%22%3A%22COUNT%22%7D%5D&f=html

Here are the first few results:

# records: 383

DFIRM_ID: 13245C
COUNT: 8286

DFIRM_ID: 06037C
COUNT: 10632

DFIRM_ID: 17031C
COUNT: 1879

DFIRM_ID: 13115C
COUNT: 2678

DFIRM_ID: 06001C
COUNT: 12788

DFIRM_ID: 04013C
COUNT: 32702

DFIRM_ID: 04025C
COUNT: 2428

DFIRM_ID: 48453C
COUNT: 60419

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