EPSG:10618

proj4.defs("EPSG:10618","+proj=vgridshift +grids=ca_nrc_HT2_2002v70_CGG2013a.tif +multiplier=-1");

{
    "$schema": "https://proj.org/schemas/v0.7/projjson.schema.json",
    "type": "Transformation",
    "name": "CGVD28(HTv2.0) height to CGVD2013a(2002) height (1)",
    "source_crs": {
        "type": "VerticalCRS",
        "name": "CGVD28(HTv2.0) height",
        "datum": {
            "type": "VerticalReferenceFrame",
            "name": "Canadian Geodetic Vertical Datum of 1928 (Height Transformation version 2.0)",
            "anchor_epoch": 1997
        },
        "coordinate_system": {
            "subtype": "vertical",
            "axis": [
                {
                    "name": "Gravity-related height",
                    "abbreviation": "H",
                    "direction": "up",
                    "unit": "metre"
                }
            ]
        },
        "id": {
            "authority": "EPSG",
            "code": 10588
        }
    },
    "target_crs": {
        "type": "VerticalCRS",
        "name": "CGVD2013a(2002) height",
        "datum": {
            "type": "VerticalReferenceFrame",
            "name": "Canadian Geodetic Vertical Datum of 2013 (CGG2013a) epoch 2002",
            "anchor_epoch": 2002
        },
        "coordinate_system": {
            "subtype": "vertical",
            "axis": [
                {
                    "name": "Gravity-related height",
                    "abbreviation": "H",
                    "direction": "up",
                    "unit": "metre"
                }
            ]
        },
        "id": {
            "authority": "EPSG",
            "code": 20034
        }
    },
    "interpolation_crs": {
        "type": "GeographicCRS",
        "name": "NAD83(CSRS)v4",
        "datum": {
            "type": "GeodeticReferenceFrame",
            "name": "North American Datum of 1983 (CSRS) version 4",
            "anchor_epoch": 2002,
            "ellipsoid": {
                "name": "GRS 1980",
                "semi_major_axis": 6378137,
                "inverse_flattening": 298.257222101
            }
        },
        "coordinate_system": {
            "subtype": "ellipsoidal",
            "axis": [
                {
                    "name": "Geodetic latitude",
                    "abbreviation": "Lat",
                    "direction": "north",
                    "unit": "degree"
                },
                {
                    "name": "Geodetic longitude",
                    "abbreviation": "Lon",
                    "direction": "east",
                    "unit": "degree"
                }
            ]
        },
        "id": {
            "authority": "EPSG",
            "code": 8246
        }
    },
    "method": {
        "name": "Vertical change by geoid grid difference (NRCan)",
        "id": {
            "authority": "EPSG",
            "code": 1126
        }
    },
    "parameters": [
        {
            "name": "Geoid model difference file",
            "value": "HT2_2002v70_CGG2013a.byn",
            "id": {
                "authority": "EPSG",
                "code": 1063
            }
        }
    ],
    "accuracy": "0.02",
    "scope": "Change of height to a different vertical reference surface.",
    "area": "Canada - onshore and offshore between 41°N and 84°N and west of 48°W - Alberta; British Columbia; Manitoba; New Brunswick; Newfoundland and Labrador; Northwest Territories; Nova Scotia; Nunavut; Ontario; Prince Edward Island; Quebec; Saskatchewan; Yukon.",
    "bbox": {
        "south_latitude": 41,
        "west_longitude": -141.01,
        "north_latitude": 84,
        "east_longitude": -48
    },
    "id": {
        "authority": "EPSG",
        "code": 10618
    },
    "remarks": "Equal to HT2_2002v70 hybrid geoid model minus CGG2013a geoid model (CT 10610 minus CT 10110). Specified interpolation CRS is NAD83(CSRS)v4, but NAD83(CSRS)v2 or any later realization may be used without error. Also used as CT 10519 from CGVD28 height."
}

<?xml version="1.0" encoding="utf-8"?>
<Map srs="+proj=vgridshift +grids=ca_nrc_HT2_2002v70_CGG2013a.tif +multiplier=-1">
	<Layer srs="+proj=vgridshift +grids=ca_nrc_HT2_2002v70_CGG2013a.tif +multiplier=-1">
	</Layer>
</Map>

INSERT into spatial_ref_sys (srid, auth_name, auth_srid, proj4text, srtext) values ( 10618, 'EPSG', 10618, '+proj=vgridshift +grids=ca_nrc_HT2_2002v70_CGG2013a.tif +multiplier=-1', 'None');